Comments by "Liam" (@1495978707) on "Steam Engine - How Does It Work" video.

Even if the engine is sideways, atmospheric pressure could still push it. The reason for double action is that the steam (which is at a significantly higher pressure than atmospheric) is acting on every stroke. This difference is similar to the difference in power per size from a 4 stroke engine to a 2 stroke engine. Steam locomotive engines had at least two cylinders, meaning four power strokes per revolution. This is partly for power, but it is necessary for the engine to be self starting. This means that the engine is always in a power stroke.
1