I used scalecoat stripper - Walthers number 640-56. It lists at $8.95 for a 16 oz bottle - it is reusable so it goes a long way. It should be in stock and available at any quality train shop. If not, it can be ordered.
Very easy to use -
Soak the car in the stripper for "a while" (it won't damage the plastic - I feel it actually gives the plastic a little better tooth
for paint than plain styrene) - I would say a several hours is best, but I don't think you'll hurt anything by leaving it in overnight. Use an old toothbrush and warm water to rub off the paint - it generally comes right off, but you may get a few sticky spots around windows, details, rivets etc. Read the instructions -very clear and easy to use.
My advice -
1) Get at least two bottles. The 16 oz by itself is probably not enough to cover a car - no matter what shape your container.
2) I used an old soda bottle with the top cut off. I recommend getting a smaller container that will still hold an 85' car as snugly as possible without touching the sides - e.g. a long baking dish or box-shaped tupperware etc. I had to keep re-positioning the car on its sides to get it to sit in the stripper. A smaller, square container with an extra bottle will let you submerge the whole thing at one time.