The two quickest working are Easy-Off oven cleaner, and Castrol Super Clean (Large Purple Jug). Neither of them will harm plastic, and work the fastest. On the die-cast loco shells (mostly Prewar), I have been able to strip all the paint of with Castrol Super Clean, in as little as 3 hours. Other commercial paint strippers are no where as good as the two I just mentioned.
Bead blasting is perfect, BUT not everyone has access to a place or someone who can do it, so it basically comes down to "chemical stripping". If you decide to try the Super Clean, use if full strength, and either of the two I mentioned, you can use dish soap, and water to rinse them off after the stripping is done. I also use an old toothbrush to get into the crevices and detail. Easy off is a single use, whereas the Super Clean can be used over again. I have been able to use it on up to 5 shells, before I get rid of it. I also use a "Spackling Trough, cheap plastic one" which is typically big enough to dunk a complete shell into.