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

 

[OddRails]

See if your version of Trainz includes a tutorial on driving steam locomotives. That should help.
As Peter says on steam locos that slider controls the rate that coal is added to the firebox.
I'm terrible at driving steam locos, but have been trying to learn after buying K&L's 2-6-6-0 for Coal Country 2.0