This one popped up at the local flea market.  I believe it is an emergency brake valve with whistle for a locomotive.   

The valve opens when the handle is pushed in either direction and there is a button that apparently activates the whistle. 

Oh, now that's got potential!  Imagine being able to call the kids/grandkids in for dinner with that!

I'd like to know what it sounds like with 150PSI pressure hooked to it?   

Talked to our local excursion train folks.  This is used on the end car air hose.  In an emergency, the brakeman can turn the lever, dumping all the air and stopping the train.  He can also use the whistle to signal the cab.  (must have been prior to radio communication systems).

I can remember the caboose having a whistle of some sorts. On the short line that ran into the center of town went thru  residential area. If the caboose was in the lead they would whistle away. geez almost all those sidings and small loading and unloading docks, even the small rail yards are gone.