It is not a postage stamp of Pomeroy's Letter Express. It is, rather, a reprint, (well after the fact) of a package label used by the express company (not "Letter Express") prior to 1844.The package express company did not carry letters other than so-called money letters and letters related to their freight. The label that was actually used was black on orange-red glazed paper. As a reprint, in perfect condition, maybe worth $15 dollars. In the condition you show, zero.
If you are a train thematic collector, look to Wyman's Independent Mail Company stamps and covers. The earliest train illustrated postage stamps in the world.
A nice example on cover is here: http://www.philamercury.com/viewcov...=24135&num=1