Someone somewhere may have a old tube type cab signal system that needs these. Probably rare as hen teeth but still overpriced, these were made as US&S OEM parts it appears.
http://www.ebay.com/itm/New-TungSol-USS308-Black-Plate-Triode-Vacuum-Tube-/151549575913
I have no connection to the seller, I just happened to 'trip' over this listing.

I'm trying to think what systems would use a Nixie tube like display, most early systems showed separate position light signal displays back lit from a light.

These are not nixes, they are amplification triodes. Their application seems to be to pick up what was a very weak signal from the tracks and amplify it to a usable level.