Mr D.
Hello & Welcome to the Forum!
My simple solution for gears slipping on a shaft is Loc tight. a drop where you want the gear to stay, move it into place and let it cure...
Not elegant, but works for me, hope it helps you.
If you decide to replace, my suggestion, if you don't have a LHS is to contact forum member Bill Hennings' family shop.
http://www.henningstrains.com/Happy Rails...
dave