There had better not be any air in a steam locomotive’s cylinders! When working superheated steam, the temperature in the cylinders is high enough to ignite the valve oil if oxygen is present. Nothing ever goes into the cylinders except steam. The valve oil that lubes the valves and cylinders is atomized in the steam.
When a steam locomotive moves, either the throttle is open a little to admit steam and lube to the cylinders, or the locomotive may be equipped with Drifting Valves that allow the locomotive to move with the throttle closed and still keep the cylinders lubed.
Steam locomotives do have a “neutral” of sorts. The valve gear can be set to move the locomotive forward or backward. When the gear is “on center” between forward and reverse, it could be said to be in “neutral.” But you would not allow a steam engine to be moved with the valve gear in this position. The valve gear must be set in the direction of travel.
The “Blow Down Valve” is back on the firebox. It is used to “blow” mud, scale, and other impurities out of the boiler. The valves on the cylinders that allow water to escape are the Cylinder Cocks.
John Sethian’s point about steam locomotives being free-wheeling is a good one. There is no worm gear on a real steamer. 😉
