Real steam locos explode when the water level is too low.
Märklin mfx locos in "world of imitation" mode run faster when the water level is low. This is so unrealistic, but that is how it is done.

I think Microsoft's Train Simulator is much more realistic when it comes to operating a steam loco.

Driving a steam loco is art. Replenish water and the pressure goes down. Add fresh coal and the pressure goes done.
Train driver and fireman must know the route and add water and coal when not much steam is needed in order to have a hot fire, a full boiler and full pressure when steam is needed.
None of this is covered in the basic algorithms of mfx steam locos.

See also:
https://en.wikipedia.org/wiki/Boiler_explosion