Why a Snowdon engine looks as if it is falling over on the level.
A steam boiler has one rule that must never be broken: the top of the firebox, the crown sheet, must stay under water. Uncovered, it overheats in moments and can fail. On a steep line the water runs to the downhill end of the boiler, so mountain engines have their boilers set at a slant to the frames. Snowdon's lean forward at 1 in 11, so that on the climb they sit nearly level.
Look at the level setting: with the tilted boiler the water now runs forward, away from the firebox. That is why the crews watch the gauge glass most carefully at the stations, where the track is nearly flat.
Pilatus, at 48 %, was far too steep for a boiler laid along the car. Its steam railcars of 1889 carried the boiler across the frame, so the slope ran across the boiler's short dimension instead of along it.
The engine always works at the downhill end of the train. It pushes on the way up and holds back on the way down, so a broken coupling can never leave the coaches running away from it. On Snowdon the coaches are not coupled to the engine at all: they rest against it. In 1896 that saved them.
Sources: Engineering (1896), the Snowdon engines and the accident; H. Wismann, "Eduard Locher" (2005).