Has there existed any steam locomotive with a valve gear such that there are pistons both in front of and behind the driving wheels, connected to the same set of linking rods?

This would result in four total pistons per set of driving wheels. The front and rear pistons would alternate pushing and pulling in sequence. I imagine this might simplify valve gear function dramatically.

The Wikipedia entry on valve gears doesn't mention any such system, and the locomotive wiki's entry on the subject is even worse, not mentioning any types at all.

This article details some additional, older valve gears, but none of them resemble the type I was thinking of railwaywondersoftheworld

If such a valve gear has existed, where can I learn more about it? If not, is there a reason such a valve gear would be infeasible?

  • 3
    If the pistons are not perfectly matched, they fight and do bad mechanical things to each other. And, frankly, steam locomotives were not pinnacles of engineering precision, mostly because they did not need to be.
    – Jon Custer
    Feb 22, 2021 at 22:27

1 Answer 1


The closest thing I've been able to find to your suggested layout is the Baltimore and Ohio class N-1: a 4-4-4-4 locomotive (front and rear pistons, like you describe, but powering independent sets of wheels).

The reason your design is unlikely to ever see use is a matter of balance. Two-piston locomotives cannot be fully balanced: there will always be residual vibration from the motion of the pistons, valve gear, and main rods. This vibration limits the speed of the locomotive. Four-piston locomotives can eliminate the linear vibrations that a two-piston locomotive experiences, but only if the additional two pistons are moving opposite their counterparts. Thus, anyone going to the extra effort of putting four pistons on a locomotive is going to take the opportunity to reduce vibration.

The "pistons-at-the-rear" design you propose has an additional problem: the back of a steam locomotive is crowded. You've already got the firebox and the cab, and now you somehow need to fit two large pistons and their steam pipes as well.

The "push-pull" argument doesn't work, either. Virtually every steam locomotive ever made has used double-acting pistons, which give the benefit of continuous power without the complexity of additional pistons.

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