We eventually found it, one block over, (in a COMPLETELY different place from where the Amtrak counter told us it was) but around a really weird back way, on the 5th floor of a parking garage that was BARELY at all indicated, only really visible once you got right up to the parking garage entrance. So you had to stop and get a ticket (as if parking there for real) drive up four flights of garagespiral, park, wondering all the while if this is what you're supposed to do, find a TINY sign saying "enterprise office 1st floor", wait for an elevator to take you down there, find a TOTALLY DARK enterprise office, discover they only open at 8am. Except there are some people in the back... with no lights on but their computer monitors. One of them, after 5 minutes or so, comes out and processes our return. R a t h e r slowly.
So, really, even if we had got to the exact right place as soon as humanly possible, we probably would have missed the train for vastly underestimating how long car rental return would take.
Anyway, the good news is Amtrak let us change dates to tomorrow at absolutely no cost, and simrob, world record holder of tolerating jcreed-presence in his living-space, said we could crash there one more night.