THERE are so many things you can’t do in the big, bad city, and you’d think that one of them would be climbing down a manhole when you’re not an employee of the Department of Transportation or the Metropolitan Transportation Authority or Con Edison. But you would be wrong.

Thanks to Bob Diamond, the man who rediscovered a 19th-century Atlantic Avenue train tunnel in Downtown Brooklyn, you can scurry down a certain manhole without being hauled away by your ankles by the authorities.

Since his big reveal in 1980, Mr. Diamond, the 49-year-old founder of the Brooklyn Historic Railway Association, has been conducting tunnel tours via the manhole with the blessing of the D.O.T. But of late, Mr. Diamond has been pushing for another potential urban architectural “get.”

Behind a wall in the tunnel, near Atlantic Avenue and Hicks Street, he believes, there is a steam locomotive lying on its side like an abandoned toy train, in “pristine condition, a virtual time capsule.” And he wants to dig it up.