What do these controls do for steam engines?

[Angrysubie64]

I am new at the Trainz franchise and so far what I know about the controls, starting from the left is the reverser, next is the regulator, the train brake, and the independent brake I think, and between that and the steam injector is the control lever in question. I need some help with what is this slider and what it does.

 

[Angrysubie64]

This screenshot may be better as the lever in question is circled in blue:

 

[Forester1]

I am thinking maybe coal? But I admit I haven't run a steam session in cab mode in a while. I don't think it is sand.

 

[JCitron]

I am thinking maybe coal? But I admit I haven't run a steam session in cab mode in a while. I don't think it is sand.

That is sand, at least it is in diesel locomotives. It's been ages since I drove steam locomotives in cab mode myself.

 

[p-dehnert]

In a steam locomotive it's coal. It controls the shoveling rate of the automatic coalman.

Peter