Trains would definitely have their uses, although their reliability, like all UNSC vehicles to get the job done in terms of transport is seriously questionable.
e.g. the way pelicans frequently seem to crash in the halo series.
OT: Although it is disputed that trains may or may not of been used in the halo saga, my question is why aren't modern navy vessels used, mainly the submarine.
e.g. a submarine can go deep underwater, would the covenant have a means of dispatching these vessels? and then there's also the possibility of nuclear subs etc. which in effect could of proven to be a nightmare for the covenant...