Why Do Trains Use DC Current? Unraveling the Electrical Mysteries of Rail Transport

Have you ever found yourself staring out the window as a powerful locomotive glides by, its rhythmic hum a constant companion on countless journeys, and wondered, "Why do trains use DC current?" It's a question that might not immediately spring to mind, but for those of us fascinated by the intricate engineering that powers our modern world, it’s a genuinely intriguing one. I remember a trip I took out West a few years back, the vast plains stretching out before me, and the steady rumble of a freight train carrying its precious cargo. There was something so reliable, so fundamental about its movement. It got me thinking about the invisible force that made it all possible. While AC (Alternating Current) is the workhorse of our power grids, powering everything from our homes to our factories, a significant portion of the rail world operates on DC (Direct Current). This choice isn't arbitrary; it's a deeply rooted decision driven by a confluence of historical, technical, and operational considerations that have shaped electric rail transport for over a century. Let's dive deep into why trains, particularly many of the ones we see chugging along, rely on direct current.

The Fundamental Difference: AC vs. DC Power in a Nutshell

Before we get into the specifics of why trains opt for DC, it’s essential to grasp the core distinction between AC and DC power. Think of electricity as water flowing through a pipe. In Direct Current (DC), the water flows steadily in one direction, like a river. The electrons move consistently from the negative terminal to the positive terminal. This is the type of power you get from batteries. Alternating Current (AC), on the other hand, is like water sloshing back and forth in the pipe. The direction of electron flow reverses periodically, typically 60 times per second (60 Hz) in the United States. This constant switching is what makes AC incredibly efficient for long-distance power transmission, as its voltage can be easily stepped up or down using transformers.

So, if AC is so great for transmission, why don't all trains run on it exclusively? This is where the unique demands of powering a massive, heavy vehicle like a train come into play. The decision involves balancing efficiency, safety, cost, and the very nature of electric traction motors.

A Look Back: The Historical Roots of DC in Traction

The story of electric trains is inextricably linked to the early days of electrical engineering, a time when the competition between AC and DC was a heated debate. Pioneers like Thomas Edison championed DC power, and his early electrical systems, developed in the late 19th century, primarily utilized DC. Consequently, the first electric railways and tramways were built using DC technology. These systems were relatively straightforward to implement at the time, especially for shorter routes and urban environments where the limitations of DC transmission (voltage drop over distance) were less of a concern.

Early electric locomotives were designed to run on DC power supplied directly from an overhead catenary or a third rail. These early systems, though rudimentary by today's standards, proved the viability of electric traction. The technology was readily available, and the motors themselves were simpler and more robust in their DC configurations. As electric traction technology evolved, the advantages of DC for certain applications continued to solidify its place in the rail industry.

The Heart of the Matter: Traction Motors and DC

One of the most compelling reasons why trains use DC current lies in the inherent advantages of DC motors for traction applications. Historically, DC motors, particularly series-wound DC motors, were the go-to choice for electric locomotives and multiple units. These motors offered excellent starting torque, which is absolutely critical for getting a heavy train moving from a standstill. Imagine the immense force needed to overcome inertia and friction for a train weighing hundreds, even thousands, of tons. DC series motors excel at providing this high torque at low speeds, gradually decreasing as the speed increases.

Here’s a breakdown of why DC motors are so well-suited for this role:

High Starting Torque: As mentioned, this is paramount. DC series motors provide a very strong initial push, essential for accelerating a heavy load. Their torque is roughly proportional to the square of the armature current, meaning a little more current yields a lot more torque when you need it most.
Simplicity and Robustness: Early DC traction motors were relatively simple in design and incredibly durable. They could withstand the harsh conditions of railway operation – dust, vibration, and temperature fluctuations – with impressive reliability. This robustness was a significant factor in their widespread adoption.
Ease of Speed Control (Historically): While modern AC motors offer sophisticated speed control, historically, controlling the speed of DC motors was more straightforward. Techniques like varying the voltage supplied to the armature or weakening the field flux allowed for relatively simple speed adjustments. This was crucial for controlling train speed in various operational scenarios.

It's important to note that while AC motors have advanced significantly and are now widely used, particularly in modern high-speed trains (often utilizing sophisticated inverters to convert AC to variable frequency AC for the motors), the legacy and continued application of DC motors in many rail systems are a testament to their enduring capabilities, especially in urban and heavy-duty freight applications.

Powering the Infrastructure: The Third Rail and Catenary Systems

The method of delivering electrical power to the train is another critical factor. The two primary methods for electric trains are the third rail and the overhead catenary system, both of which are historically and practically linked to DC power distribution.

The Third Rail System

You’ve likely seen this in action in subway systems or some commuter rail lines. A “third rail,” typically positioned alongside or between the running rails, carries the electrical current. The train picks up this current via a “shoe” that makes contact with the third rail.

DC Dominance: For many years, third rail systems were exclusively powered by DC. This was due to the simpler nature of DC power substations, which could efficiently convert high-voltage AC from the grid to lower-voltage DC (often around 600-750 volts) for direct supply to the third rail.
Advantages: Third rail systems are generally more aesthetically pleasing than overhead wires, as they are less visually intrusive. They are also often less expensive to install in tunnels and urban areas where the construction of tall poles and extensive overhead wiring might be impractical or prohibitively costly.
Disadvantages: The primary drawbacks of third rail systems include safety concerns (the live rail can be a hazard to the public if not properly protected), limitations in voltage (higher voltages increase insulation challenges and electrical arcing risks), and potential issues with ice or debris accumulation on the rail, which can interrupt power collection. These limitations naturally steered towards lower-voltage DC systems.

The Overhead Catenary System

This is the more common system you see on intercity and high-speed rail lines. A network of wires, suspended above the tracks by poles or gantries, carries the electrical current. The train collects this current via a pantograph, a device mounted on the roof that presses against the catenary wire.

AC and DC Applications: While AC power is more commonly associated with modern, high-voltage catenary systems (often operating at 25 kV or higher), DC power has also been extensively used, particularly in older systems or for specific applications. Systems operating at lower voltages, such as 1500V DC or 3000V DC, are common in many parts of the world, including parts of Europe and Asia.
Advantages: Catenary systems can deliver power at much higher voltages than third rail systems. This is crucial for long-distance transmission and for powering high-speed trains. Higher voltage means less current is needed for the same amount of power, reducing energy loss due to resistance in the wires.
Historical Preference for DC: In the development of early catenary systems, DC was often the preferred choice due to the simpler power conversion and control equipment available at the time. Many older DC catenary systems still operate today, providing reliable service.

The choice between third rail and catenary, and the type of current used within them, often depends on the specific operational requirements, historical context, and economic considerations of a particular railway network.

Voltage Drop and Power Transmission Challenges

This is where the distinction between AC and DC becomes particularly relevant for the scale of railway operations. Powering a train, especially a heavy freight train or a long passenger train, requires a tremendous amount of energy. Delivering this energy efficiently over distance is a significant engineering challenge.

DC Power Transmission:

Voltage Limitation: One of the primary limitations of DC power is that it is inherently difficult to change its voltage efficiently over long distances. While AC voltage can be stepped up to very high levels for transmission (reducing current and thus resistive losses) and then stepped down for use, DC voltage remains relatively constant.
Substations and Proximity: To overcome the voltage drop inherent in transmitting DC power over long distances, railway operators need to install numerous substations at regular intervals along the track. These substations convert the high-voltage AC from the national grid to the required DC voltage. The closer these substations are, the shorter the distance the low-voltage DC has to travel, minimizing power loss. This can significantly increase infrastructure costs and complexity.
Third Rail Vulnerability: For third rail systems, the DC voltage is typically much lower (e.g., 600-750V), making voltage drop even more pronounced. This is why third rail systems are generally confined to urban or suburban areas where the distances are relatively short.

AC Power Transmission (and its application in trains):

Efficient Transmission: AC power can be easily transformed to very high voltages (e.g., 11kV, 25kV, 50kV and above) for efficient transmission over long distances with minimal losses. This is why the national power grids use AC.
Modern AC Traction: In many modern electric trains, AC power from the overhead catenary is the initial form of power. However, this AC power is then often converted onboard the train (or at substations) into DC, and then further converted into variable-frequency AC for the traction motors. This complex process, known as Variable Voltage Variable Frequency (VVVF) control, allows for highly efficient and precise control of AC traction motors, offering superior performance and regenerative braking capabilities compared to older DC systems.

So, while AC is superior for *transmission* to the railway line, the *onboard* or *direct supply* to certain types of traction motors has historically favored DC. The evolution of power electronics has blurred these lines, allowing for the efficient use of AC power even in systems that ultimately utilize DC components or motors.

Safety Considerations

Safety is always a paramount concern in railway operations, and the choice of current type plays a role in this. While both AC and DC have their own safety considerations, DC has historically offered some advantages in certain contexts.

DC Shock Hazard: While DC can deliver a significant shock, the nature of the shock can differ from AC. DC current tends to cause muscle contraction, making it difficult to let go of a live conductor. AC, particularly at mains frequencies, can cause more severe tissue damage and cardiac arrest. However, the voltages used in many DC traction systems (like third rails) are low enough that the immediate danger of electrocution is somewhat mitigated, though still a serious risk.
Third Rail Protection: As mentioned, third rail systems, often DC, pose a physical hazard. Rail operators employ various measures to mitigate this, including insulating covers, raised platforms, and signage, but the inherent risk remains.
Overhead Catenary Hazards: Overhead catenary systems, regardless of whether they carry AC or DC, present a significant electrical hazard due to the high voltages involved. Extreme caution is required around these systems, and unauthorized access is extremely dangerous.
Arcing and Faults: DC systems, especially at higher voltages, can experience arcing when contact is lost or during fault conditions. This arcing can be dangerous and can damage equipment. However, AC systems also have their own arcing and fault issues, particularly at high voltages and frequencies.

Ultimately, safety protocols and robust engineering design are far more critical than the specific type of current used. Both AC and DC systems can be operated safely when properly designed, maintained, and operated with strict adherence to safety regulations.

Cost and Complexity of Infrastructure

The decision of why trains use DC current is also heavily influenced by the economics of building and maintaining a railway infrastructure.

Early Simplicity: In the early days of electric traction, DC technology was simpler and less expensive to implement than the more complex AC systems that were still being developed. This cost advantage was a significant driver for adopting DC for many urban and suburban rail lines.
Substation Costs: As discussed, DC systems require more substations to compensate for voltage drop over distance. The cost of building and maintaining these numerous substations can be substantial. However, the alternative of high-voltage AC transmission requires more sophisticated and expensive catenary infrastructure and more complex onboard equipment to convert and control the power.
Third Rail vs. Catenary: Third rail systems, often DC, can be cheaper to install in tunnels and dense urban environments compared to overhead catenary systems. This has made DC a practical choice for many metro and underground railway systems worldwide.
Maintenance: The maintenance requirements for DC motors and associated switchgear have historically been considered manageable, contributing to their long-term viability.

The economic calculus is always evolving with technological advancements. Modern power electronics have made AC traction systems more competitive, but the established DC infrastructure in many regions continues to be a powerful reason for its continued use. Replacing an entire DC infrastructure with an AC one would be a colossal undertaking, both financially and logistically.

Regenerative Braking and Energy Efficiency

Regenerative braking is a critical technology for improving the energy efficiency of electric trains. It allows the train's motors to act as generators during braking, converting the train's kinetic energy back into electrical energy that can be fed back into the power supply system.

DC Systems and Regeneration: Regenerative braking is indeed possible with DC traction systems. When the DC motors are switched to act as generators, they produce DC power. This DC power can then be fed back to the DC supply line, where it can be used by other accelerating trains or dissipated in resistors if there is no demand.
Challenges with DC Regeneration: A key challenge with DC regenerative braking, especially in third rail systems or lower-voltage DC catenary systems, is the inability to easily "push" this regenerated power back into the high-voltage AC grid. The generated DC power needs to be compatible with the existing DC supply voltage. If there are no other trains consuming power or if the regenerated voltage doesn't match the supply voltage, the energy must be absorbed by resistors, leading to energy waste as heat.
AC Systems and Grid Interaction: Modern AC traction systems, particularly those using sophisticated inverters and converters, are far more adept at managing regenerative braking. They can convert the generated DC (after rectification from the AC motors) into AC power at the correct voltage and frequency to be fed back into the main AC grid. This seamless integration with the grid allows for much higher levels of energy recovery and efficiency.

While regenerative braking is possible with DC, the ability to efficiently return energy to the grid is significantly enhanced with modern AC systems. This is a growing factor in the shift towards AC in new rail electrification projects.

The Dominance of AC in High-Speed Rail

When we talk about modern, high-speed rail, the story shifts dramatically towards AC. Why? The answer lies in the fundamental advantages of AC for high-power, high-speed applications.

Efficient Power Delivery: High-speed trains require immense amounts of power to overcome air resistance and friction at extreme speeds. AC power, transmitted at very high voltages (e.g., 25 kV AC), allows for efficient delivery of this power to the train with minimal losses in the overhead catenary.
Advanced AC Motors: Modern AC induction motors and permanent magnet synchronous motors, controlled by sophisticated Variable Voltage Variable Frequency (VVVF) inverters, offer unparalleled performance for high-speed traction. They provide excellent power-to-weight ratios, precise speed control, and highly efficient regenerative braking.
Simplified Infrastructure: Although the onboard electronics are complex, the overhead catenary infrastructure for high-voltage AC is often simpler and requires fewer substations than equivalent high-power DC systems. This makes AC the more practical and economical choice for long-distance, high-speed lines.
Flexibility and Control: VVVF inverters allow for precise control over motor speed and torque, enabling smooth acceleration and deceleration, which is vital for passenger comfort at high speeds. They also facilitate the efficient implementation of regenerative braking that can feed energy back into the grid.

Therefore, while DC remains prevalent in many established urban, suburban, and freight rail networks, AC power is the undisputed standard for new high-speed rail developments worldwide.

Hybrid Systems and the Future

It’s not always a black and white choice. Many railway networks employ hybrid systems, utilizing different power sources depending on the line section.

Dual-Mode Locomotives: Some locomotives can operate on both DC third rail or overhead line and diesel power, allowing them to travel on electrified lines and then switch to diesel for non-electrified sections.
AC/DC Catenary Systems: Some overhead lines are designed to supply both AC and DC power, catering to a mix of older DC trains and newer AC trains on the same route. This requires dual-voltage pantographs and sophisticated onboard systems.
Modern Converter Technology: The advent of advanced power electronics, such as Insulated Gate Bipolar Transistors (IGBTs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), has revolutionized how electricity is managed in traction. These devices allow for efficient conversion between AC and DC at various voltages and frequencies. This means that even in systems that receive AC power, it can be converted to DC for onboard systems or for DC traction motors, or converted to variable-frequency AC for AC traction motors.

The future of rail electrification is likely to involve even greater integration and flexibility, with systems capable of seamlessly switching between power sources and optimizing energy usage. However, the fundamental reasons for the historical and continued use of DC in specific applications remain relevant.

Why Some Trains Still Use DC: A Summary

Let's distill the core reasons why you'll still find many trains relying on DC current:

Legacy Infrastructure: A vast amount of railway infrastructure, particularly in urban and older intercity networks, was built around DC power. The cost and complexity of converting this entire infrastructure to AC are prohibitive.
DC Traction Motors: For certain applications, particularly in heavy-duty freight and older urban transit systems, DC series-wound motors continue to be valued for their high starting torque, robustness, and relative simplicity.
Third Rail Systems: Third rail power delivery, which is common in metro and subway systems, is predominantly DC due to voltage limitations and installation practicalities.
Cost-Effectiveness in Specific Niches: In certain contexts, particularly for shorter routes or where simpler DC equipment is sufficient, DC systems can still be more cost-effective to install and maintain than complex AC systems.
Historical Development: The early development of electric traction was heavily influenced by DC technology, establishing a strong foundation that persisted for decades.

Frequently Asked Questions About Trains and DC Current

Why do subway trains use DC current?

Subway trains overwhelmingly use DC current primarily due to the prevalence of third rail power collection systems. Third rail systems, which are common in tunnels and dense urban environments, are typically powered by DC. This choice is driven by several factors:

Voltage Limitations of Third Rails: Maintaining high voltages in a third rail system poses significant safety and insulation challenges. Higher voltages increase the risk of electrical arcing and physical contact hazards for passengers and maintenance workers. Therefore, lower voltages, like 600-750 volts DC, are generally used.
Substation Design: DC substations for third rail systems are relatively straightforward. They take AC power from the national grid, convert it to the required DC voltage, and supply it directly to the third rail. The need for numerous substations spaced relatively close together to manage voltage drop is a consequence, but the technology for these substations is well-established and understood.
Traction Motor Characteristics: Historically, DC traction motors provided the necessary high starting torque for accelerating heavy subway trains from a standstill, a critical requirement in frequent stop-and-go operations. While AC motors have advanced significantly, the existing fleet of DC-powered trains and their associated infrastructure continue to justify DC usage.
Cost and Simplicity in Confined Spaces: Installing and maintaining third rail systems can be more cost-effective and practical in tunnels and underground environments compared to the extensive overhead catenary required for high-voltage AC transmission. The overall system complexity for DC third rail is often considered lower for these specific applications.

In essence, the combination of safety considerations, historical precedent, motor technology, and infrastructure practicalities has cemented DC power as the dominant choice for most subway systems worldwide.

Are all electric trains DC?

No, absolutely not all electric trains are DC. While DC current is used in a significant portion of electric railway networks, particularly in urban transit, older intercity lines, and freight operations, Alternating Current (AC) is increasingly prevalent, especially in modern applications.

Here's a more nuanced look:

AC Dominance in High-Speed Rail: Modern high-speed trains overwhelmingly utilize AC power. This is because AC can be transmitted at very high voltages (e.g., 25 kV AC) over long distances with minimal energy loss, a critical factor for powering trains that travel at speeds exceeding 150-200 mph.
Advanced AC Traction Systems: Today's AC electric locomotives and multiple units often use sophisticated onboard electronics (inverters) to convert the incoming AC power into variable-frequency AC. This allows for precise control of AC traction motors, offering superior performance, efficiency, and regenerative braking capabilities.
Hybrid Systems: Many railway networks operate with a mix of AC and DC technologies. Some lines might have overhead wires that can supply both AC and DC power, accommodating different types of trains. Furthermore, modern electric trains might receive AC power from the overhead line and then convert it onboard to DC for specific subsystems or even to DC traction motors, blurring the lines between pure AC and DC systems.
DC's Continued Relevance: Despite the rise of AC, DC remains vital in many established networks due to legacy infrastructure, the suitability of DC motors for certain traction tasks (like high starting torque), and the cost-effectiveness of third rail DC systems in urban environments.

So, while you might still see and hear many trains running on DC, the landscape of electric traction is diverse, with AC playing a progressively larger role, especially as new technologies and high-speed networks are developed.

How is DC power supplied to trains?

DC power is supplied to trains through two primary methods, each with its own infrastructure and operational characteristics:

Third Rail System:
- Infrastructure: A dedicated conductor rail, known as the "third rail," is installed alongside or between the running rails. This rail is energized with DC electricity, typically at voltages ranging from 600 to 1500 volts DC.
- Power Collection: Trains are equipped with "collector shoes" or "contact shoes" mounted on their undercarriages. These shoes slide along the third rail, making continuous electrical contact to draw the DC power needed to operate the traction motors and other onboard systems.
- Power Source: The DC power for the third rail is supplied by numerous substations strategically located along the railway line. These substations convert high-voltage AC power from the national electricity grid into the lower DC voltage required for the third rail. The proximity of these substations is crucial to minimize voltage drop over the short distances typical of third rail operation.
- Common Applications: This system is most commonly found in urban metro and subway systems, as well as some commuter rail lines, where the relatively short distances and enclosed environments make it a practical and often more cost-effective solution than overhead catenary.
Overhead Catenary System (DC Variants):
- Infrastructure: While often associated with AC power, overhead catenary systems can also supply DC power. In these systems, one or more wires are suspended above the tracks. For DC systems, voltages typically range from 1500 to 3000 volts DC.
- Power Collection: Trains are equipped with a "pantograph," a retractable frame mounted on the roof, which makes contact with the overhead wire(s) to collect the DC current.
- Power Source: Similar to third rail systems, DC substations convert AC grid power to the necessary DC voltage for the catenary. These substations may be spaced further apart than those for third rail systems due to the higher voltages involved, which lead to less voltage drop.
- Common Applications: DC catenary systems are found in various regions for intercity, regional, and even some heavy-duty freight lines, particularly in countries where DC electrification was established early on. They offer advantages over third rail for longer distances and higher power requirements.

In both scenarios, the DC power is used to drive DC traction motors or is converted onboard to other forms of power (like AC) for more advanced traction systems, depending on the train's design.

What are the advantages of using DC current for trains?

The advantages of using DC current for trains, particularly in specific operational contexts, are multifaceted:

High Starting Torque: DC series-wound traction motors are renowned for their exceptional starting torque. This is critically important for getting heavy trains, laden with passengers or freight, moving from a standstill. The torque produced by these motors is directly proportional to the current, allowing for a powerful initial acceleration.
Simplicity and Robustness of DC Motors: Historically, DC traction motors have been simpler in design and more robust than their AC counterparts. They are well-suited to the harsh, vibration-prone environment of railway operation and can withstand significant abuse. This reliability has been a major factor in their widespread adoption.
Ease of Speed Control (Historically): While modern AC motor control is highly sophisticated, the speed of DC motors can be controlled relatively simply by adjusting the armature voltage or by weakening the field flux. This offered a practical way to manage train speed in earlier systems.
Suitability for Third Rail Systems: DC power is the standard for third rail systems, which are prevalent in urban environments like subways. The lower voltages typically used in third rail systems (600-750V DC) are more manageable from an electrical insulation and safety perspective than attempting to transmit AC at equivalent power levels.
Established Infrastructure: Many railway networks have extensive existing infrastructure designed for DC power. The cost and logistical challenges of converting this entire system to AC are immense, making it more practical and economical to continue using DC where it is already in place.
Simpler Power Substation Design (for DC supply): While more substations are needed to compensate for voltage drop, the individual DC substations themselves can be simpler in design and operation compared to the complex frequency converters required for some AC traction systems.

While AC power offers advantages in high-voltage transmission and advanced motor control, DC continues to be a practical and effective choice for many railway applications due to these inherent strengths.

What are the disadvantages of using DC current for trains?

Despite its advantages, the use of DC current for trains also comes with notable disadvantages:

Voltage Drop Over Distance: This is perhaps the most significant drawback. DC voltage cannot be easily stepped up to very high levels for efficient transmission over long distances like AC can. As DC power travels along the rails or through the third rail, its voltage gradually decreases due to resistance. This voltage drop means that for longer routes, numerous substations are required at relatively close intervals to re-boost the voltage. This increases the cost and complexity of the infrastructure.
Limited Transmission Efficiency: Consequently, transmitting DC power over long distances is less efficient than transmitting AC power at high voltages. More energy is lost as heat in the conductors, leading to higher operating costs.
Difficulty in Power Factor Correction: DC power systems do not inherently have a power factor issue in the same way AC systems do. However, the overall efficiency of the system is impacted by the need for more robust and larger conductors to carry the higher currents required at lower voltages over distance.
Limited Regeneration into the Grid: While regenerative braking is possible with DC systems, feeding the regenerated DC power efficiently back into the high-voltage AC national grid can be problematic. It often requires complex conversion equipment or the energy must be dissipated as heat if there is no immediate demand on the DC line. Modern AC systems are generally much better at capturing and returning regenerated energy to the grid.
Complexity of High-Power AC Motor Control (Historically): While AC motors are now dominant in high-speed rail due to advancements in power electronics, historically, achieving efficient and precise speed control for AC traction motors was more complex than for DC motors. This complexity favored DC in earlier eras.
Safety Concerns with Third Rail: While the voltages are often lower, the physical proximity of a live third rail presents a direct contact hazard to people and animals, necessitating robust safety measures and limiting its use in areas where public access is uncontrolled.

These disadvantages are key reasons why AC power, with its high-voltage transmission capabilities and advanced motor control, has become the standard for modern high-speed rail and is increasingly being adopted for other applications.

Conclusion: A Pragmatic Choice Driven by History and Application

So, to circle back to our initial question, "Why do trains use DC current?" the answer is a blend of historical momentum, specific technological advantages, and practical infrastructure considerations. For decades, DC power and its associated DC traction motors offered the most reliable, robust, and cost-effective solution for electric rail transport, especially for urban transit and applications requiring high starting torque. The legacy infrastructure built around DC systems is vast, and the expense of converting it to AC is often prohibitive. While AC power reigns supreme in high-speed rail due to its superior transmission efficiency and the capabilities of modern AC motors, DC continues to serve millions of passengers and move countless tons of freight daily, a testament to its enduring utility in the world of trains.

Why Do Trains Use DC Current? Unraveling the Electrical Mysteries of Rail Transport