There are two ways I use to remove the paint (and yes, remove the paint because old paint and decals can come through a freshly painted shell) One way is to soak the shell (and i mean submerge it) in 92% alcohol and use a nylon brush (like an old tooth brush) to remove the paint. This takes awhile but does a good job of removing the paint without harm to the plastic. The other method i use is to use Poly S paint and decal remover. I'll liberally apply it to the shell with a 1" paint brush and let it sit awhile. then i'll use a nylon brush to remove the paint. Keep appling the paint and decal remover over and over again until those stubborn areas are free from paint.
Once all the paint is gone, i'll wash the shell in warm soapy water and let it air dry overnight. Then i'll spray it with primer, let it dry overnight, then the next day, start the paint job. NS locomotives are pretty easy to paint and decal since the color is engine black with all white decals. i'll paint the climb aboard handrails yellow with a small artists brush once it's all done.
Another helpful hint is to wear a pair of surgical gloves when handling the shell after painting it. The oil on your hands can take the fresh paint off when you start handling it alot when you do the decals.