Trains and Railroads

Diesel engines are internal combustion engines in which fuel oil is injected directly into the cylinder head and ignited by pressure. They power the wheels by direct gearing rather than the connecting rods of the steam locomotive, providing continual power. Railway diesels have been designed with electric, hydraulic, mechanical, and pneumatic transmissions; today the diesel-electric engine is most common.

When they were introduced early in the twentieth century, diesels offered unprecendented efficiency and performance over steam locomotives. Diesel engines could be operated round the clock, without timeouts to take on water for the boiler or clean out ashes from the firebox. They could carry enough fuel for a day or two of continuous operation, and running them was almost absurdly simple. Crewmen for the first diesel locomotive in the United States, for example, were trained to operate it in just 15 minutes. Initial capital outlay was high, but operating costs were only a fraction of the cost of steam locomotives.

Electric trains are the other major type of motive rail power. Particularly in Europe, passenger traffic is dominated by electric power.

Electric trains run on both direct and alternating current, with voltage in the 50 to 100 kV range. The choice of current type is driven as much by economics as by performance, involving as it does a tradeoff of cost and efficiency. Alternating current (AC) offers an economical current supply at the expense of motor complexity. Motors for the more expensive direct current (DC) supplies are very simple. Current is fed to the motors from overhead wires, as with trolleys, or from an electrified "third rail" along the ground, commonly seen in subways.