I have been on small trains (most recently: Eizan railway in Kyoto) that indeed bear messages along the line of 'ワンマン', followed by information on how/where to board/alight the train (the 出入口 part).
For example, Eiden asks passengers to exit through the front door, due to the train being 'ワンマン'.
Eiden translates ワンマン as conductorless (a somewhat ungrammatical but easily understandable word), which might sound counter-intuitive, since the word is obviously wasei-eigo for one-man, not zero...
This is probably due to the fact that, even without a conductor, the train still usually has one person attending to the doors and tickets.
Although all the other meanings mentioned in this thread are valid translations of this term, given context, I think "conductorless" (or equivalent) is the most likely one.