Is there a sticker &/or number on the motor?  A circuit board? It shouldn't really be too hard to determine a replacement if the exact answer isnt known by someone.

It should take about 6v to blow soft, 9-14v normal, because that's where most trains like to run, right up to about 25v to handle being run off of track power. (instructions could be a clue, is track power mentioned or is it suggested to use aux. power only?) Lots of motors use the same cases, but the operating range varies a lot.  This could be an opportunity for tayloring it better to your needs.

The rpm needed at X-volts the main factor, voltage can more of a varible factor based on what you use (exception might be board driven) RPM because you have minimum and maximum airflow needs. Too much or too little and you won't get sound, or maybe not one you like (although I've not heard a toy train whistle pitch out yet, they seem to just get louder with more air, there is likely a limit)

You might want to check out tenders that use can motors for a repacement. The rpm and voltage range is likely close enough.

Great call Banjoflyer ..... And might be able to suggest something if they don't.  They were great help whenever I talked with them (Michigan's finest IMO!)