When I was doing consulting work, I worked with two different companies that made lithium battery packs for aerospace use. I got to see two different tests where the battery was shorted to simulate a system fault. I only wish I could have kept the video clips, they were spectacular! The batteries involved have a microprocessor with several sensors for each cell in the battery, and an additional microprocessor to manage all the individual cells.
The most spectacular was a 100 AH seven cell lithium battery for aircraft use. The test was to short one of the seven cells to force thermal runaway. The resultant carnage had to be contained inside the battery case with only a small gas vent to the outside to relieve pressure. It didn't exactly work out that way. The resulting explosion damaged the test chamber sufficiently that it took a few weeks to fix the instrumentation so it could be used again!
If you think that a little battery in your model train is dangerous, try a similar test with a wet cell lithium battery the size of a large car battery!
