It shouldn't be too difficult to remove & replace the wheel.
If you do attempt the repair, make sure that when you press on the replacement
wheel, you place it in exactly the same position as the one removed.
This will keep your wheels in quarter.

A small wheel/gear puller to remove the wheel & a press to attach the replacement.
I suspect there is a train specific puller. Check with Lionel parts & service for
one or  Micro-Mark.      http://www.micromark.com/