It's somewhat of a mystery, I have some that have stood the test of time, and others that ran, and a few weeks later were DOA. I know that I've seen a number where capacitors failed, but lots of other failures as well. Many people report storing them away working, and when they are unpacked again, they are either simply dead, or they present a direct short across the track.
A poly switch is way too slow to protect against stuff like power surges, however a TVS right in the locomotive is a good idea if you're already in there fixing things.
Another thing to do is make sure the battery is good and charged before starting up if they've been stored for some period of time.
