Does anyone know what this is? I found it in an abandoned steel mill supply room alongside parts as diverse as overhead crane controls and steam hammer piston rings. I am not even sure if this is railroad related, but it has B&O cast into it. Any ideas?

Mike,
IMHO, it is an interlocked valve for a steam hammer. The steam pilot valve is inside the boxlike housing with the pipe ports. It is essentially a reversing valve. It is interlocked with a shoe on the left that catches the hammer at the top of the stroke and holds it in the up position. The handle works both the catch shoe and the valve. A really good operator could work a 200 ton hammer like we would work a 2 Lb. ball pein.
http://www.homesteadvalve.com/