Motive Power

1. Motive power for trains is provided either by a locomotive or is incorpo­rated into a car that also has passenger, baggage, or freight accommodations. A locomotive is a self-propelled vehicle used for hauling railroad cars on tracks. Today there are two main sources of power for a locomotive: diesel fuel and electricity. Steam, the earliest form of propulsion, was in almost universal use until about the time of World War II.

2. Like the steam locomotive the diesel carries its own fuel supply, but the diesel-engine output cannot be coupled directly to the wheels; instead, a mechanical, electric, or hydraulic transmission must be used.

3. By the end of the 1960s, diesel had al­most completely replaced steam as the standard railroad motive power on nonelectrified lines in most parts of the world. Compared with steam, the diesel traction unit had a number of major advantages:

· It could operate for long periods with no lost time for maintenance.

· It used less fuel energy than a steam locomotive, for its thermal efficiency was about four times as great.

· It could accelerate a train more rapidly and operate at higher sustained speeds with less damage to the track.

4. In addition, the diesel locomo­tive had smoother acceleration, greater cleanli­ness, standardized repair parts, and operating flexibility (a number of diesel units could be combined according to the power needed for a particular train with only one crew required for all the units).

5. In the 1990s 3, 500 to 4, 000 horsepower was universally regarded as the maximum out­put to be sought from a single diesel locomotive. Several types used in Russia have much higher outputs because each locomotive is formed of two identically powered and wheeled units permanently short-coupled together (the same princi­ple has been applied to the majority of its electric locomo­tives).

6. As compared with an electric locomotive, however, the diesel has an im­portant drawback: it can develop less horsepower per locomotive unit. Because high horsepower is required for high-speed operation, the diesel is, therefore, less desirable than the electric for high-speed passenger services. The diesel locomotives excel in railroad switching where intensity of use is high.

7. Unlike steam and diesel locomotives, which produce their own power, electric locomotives simply convert the electric power that is generated elsewhere. The electric locomotive is not self-sufficient; it picks up current from an overhead wire or a third rail beside the running rails. Electric-traction systems can be divided into those using alternating current and those using direct current.

8. Electric traction is gen­erally considered the most economical and efficient means of operating a railroad, provided that cheap electricity is available and that the traffic density justifies the high capital cost. Electric locomotives have sev­eral advantages. They can develop greater power to start a heavy train or to surmount a steep grade at high speed. Electric locomotives are cleaner and quieter in operation than other types. Electric locomotives require little time in the shop for maintenance, their maintenance costs are low, and they have a longer life than diesels. The greatest drawback of electrification is the high capital investment and maintenance cost of power substations, overhead wires and structures. Electric locomotives are used principally on main-line railroads to haul standard passenger and freight equipment.

9. Railroads have experi­mented with other types of motive power. In the 1950s and '60s the gas turbine was adopted by one American and some European railroads as an alternative to the diesel engine, but its advantages were nullified by advances in diesel traction technology.