converting 8et air to 26l

We have a 1944 Baldwin 4-8-4 (SF2926)that has a cut out cock? under the engineers seat that connects the drivers brake cylinder pipe to the tender BC pipe. Id like to know what it does?

All I can say is take it apart and see how it works. It may be a way for the other engine and tender trucks to come on, but not the drivers. Maybe in the mountains they wanted some additional braking power, but didn't want to risk heating up the driver tires, which might them come off.