Steam engine is fabricated and mounted on the locomotive to give propulsion power to it. A steam engine is a heat engine that makes use of the thermal energy that exists in steam, for converting it to mechanical energy.
Tender is a small wagon on wheels independent of the main locomotive always attached to it by coupling. This is designed to carry water for the boiler and also coal fuel with sloping floor towards the driver cabin for easy shoveling of coal.
For various parts of locomotive, please see Steam locomotive nomenclature
Steam engines were used in pumps, locomotives, steam ships and steam tractors, and were essential to the Industrial Revolution. This principle of conversion of heat energy to mechanical energy is still used on a large scale for electrical power generation. However, instead of steam engines these use steam turbines.
After the development of pressurized steam technology, the next major advance was the use of double-acting pistons, with pressurized steam admitted alternately to each side while the other side is exhausted to the atmosphere. Most reciprocating engines now use this technology. Power is transmitted to the wheels by a sliding rod coming out of the cylinder, sealed against the escape of steam by provision of gland packings. This rod in turn (via a sliding crosshead bearing) is connected to a crank to convert the reciprocating engines motion to rotary motion. An additional crank or eccentric is used to drive the valve gear to control the admission of steam to one or the other side of cylinder, usually through a reversing mechanism to allow reversal of the rotary motion.
When a pair of double acting piston is used, their crank phasing is offset by 90 degrees of angle. This is called quartering. This ensures that the engine will always operate, no matter what position the crank is in.
Sand box, one each on either side is provided to feed sand in front of the wheels to avoid slipping while starting from rest.
The boilers are generally fire tube boiler type with hand feeding of coal from the tender. The ash also is removed from the grate manually. The boiler is fully lagged or covered by heat insulation. Water stored in the tender is fed by steam injectors operated by the Loco driver.
Two boiler pressure gauges are provided in the cabin prominently. Two water level gauge glasses are provided at driver's cabin with facility for water feeding by injector and operation of the safety valves manually if required. Water blow down facility from the boiler is also provided in the cabin to blow down the boiler to adjust the water level.
Generally braking system is of vacuum brake system. In this system the brake is released when the vacuum in the system is present. If the vacuum fails the brake is applied. The loco driver has the control and monitoring of the vacuum brake system. The vacuum is created by steam ejectors operated by the loco driver. The brake pads are generally of cast iron block rubbing against the wheel. Cast iron is used because of its earlier wear against that of rim of the wheel and cheaper replacement.
Steam locomotives are generally advantageous at higher elevations as they are not especially adversely affected by the lower atmospheric pressure as in the case of diesel locos or electric locos. Same analogy applies in the case of higher gradients such as up hills.
Even today the steam locomotives are used specifically in hilly regions. They are now a days designed with latest technology in instrumentation, controls and also in mechanical designs and fabrication technology. This enhances safety aspects. Further the loco driver has more easy controls and comforts for operation.
In many countries, steam locomotives are making a bit of a comeback as "museum-pieces" doubling as excursion train haulers.