the cylinders

The high pressure steam which is controlled by the regulator in the dome is piped to the cylinders which in turn drive the wheels. More early designs often had the cylinders mounted between the wheels which made maintenance much harder. More simple designs use slide valves, as shown in the animation below.

Greater efficiency is obtained by the use of piston valves.

Water condensate will collect in the cylinders when the locomotive is not in motion. Water cannot be compressed to it is essential to be able to remove it. For this reason, drain cocks are fitted to the bottom of the cylinder bores, at each end.  These can be opened by a control in the cab and are always used when the loco moves off.  The clip below shows an engine move off with the drain cocks open.

At the outer end of the piston rod is the crosshead. This slides backwards and forwards guided by one or two bars. This a a major point of wear and the replaceable bearings in the crosshead are called 'slippers'.

a single slide bar portrayed in a working model

or double.

The reciprocal movement must now be transferred to the wheels.


The cylinders need to be lubricated with a special steam oil. This can be introduced by the use of a displacement lubricator. It was introduced in the United Kingdom in 1860 by John Ramsbottom. It operates by allowing steam to enter a closed vessel containing oil. After condensing, the water sinks to the bottom of the vessel, causing the oil to rise and overflow into delivery pipes. In a steam locomotive, it was often positioned in the cab where the rate of oil feed could be observed. Alternatively, two displacement lubricators (one for each cylinder) might be positioned on either side of the smokebox.

Mechanical lubricators are far better and are generally operated by a mechanical linkage attached to an appropriate position on the valve gear.

another type of mechanical lubricator