What is the best thing to use to take the decals off a locomotive
I use a bit of non-acetone fingernail polish remover and a paper towel. Be careful as the polish remover can effect the paint as well.
Lettering on model railroad stock is not applied with decals, it is printed on with paint, making it tricky at times to remove.
You may end up with a ghosted image left behind. Removing / altering a road or car number works better than trying to remove a large logo.