zedob --
Whether UP legally has the right has never been adequately challenged in the courts, as the railroad's previous targets have all done a very complete job of knuckling under. (The fact is that such lawsuits are extremely -- ha! to say the least, expensive! -- to defend, and most manufacturers take their attorneys' advice and settle.) It can -- and really should, at some point -- be argued, however, that Union Pacific long ago allowed its logos and liveries (trademarks, service marks, etc.) to become "genericized" by failing to bring action decades earlier against any alleged "infringements."
You'll note, for example, that McDonald's legal department religiously chases down any and all commercial uses of the term, "Mc", which might in any way, shape or means cause confusion with the hamburger chain, or otherwise allow their client's name to fall into the public domain. Additionally, corporations such as Levi Strauss and Xerox, for example, take steps to ensure that any written reference to "Levi's" or "Xerox machines" are properly punctuated and that they do, in fact, refer to Levi's (as opposed to any blue jeans) or Xerox machines (instead of an unspecified brand of copier). Not to do so would eventually allow their brandnames to become "generic" and unenforceable in a court of law.
Union Pacific is approximately eighty years -- dead minimum -- late in coming to the table in this manner. Additionally, there may be at least one instance (back in the 30s) where that currently-litigious railroad actually paid a toymaker (Lionel?) to come out with models of its equipment.
I'd love to see some manufacturer not wave the white feather . . .
As to your original artwork, you of course retain copyright to it regardless of whether you give it away or sell it. Were someone else then to duplicate it in some way without your consent, you'd be entitled to sue. But not after letting decades pass.