Your best bet, and possibly cheapest, is to soak them in 91% alcohol. Depending on how good the paint on them is will depend on how long they will need to soak. Once soaked use a tooth brush (an old one preferably) to rub the old paint and decals off.
The engines/rolling stock I have stripped using 91% have taken anywhere from a few minutes to a few hours before the paint will rub of using the tooth brush. Best bet is to let them soak over night, it wont hurt anything doing that.