I have had that happen also. Too much resistance in the track joiners and when a short was left across both rails... Not enough current flowing to trip the circuit breaker. Amazing!This Christmas, I set up two trains. A passenger train to run around the tree, with my River Forest station (it is O scale). And in the front parlor a train with a rocket launcher car and several gondolas with extra rockets, green and brown army men, and my homemade set of blocks made from 2X4's (3 1/2 by 3 1/2 and 3 1/2 by 7).
I am attaching two pictures. The first was taken the day after showing how the army took over the station.
During the evening, my two grandsons (1st and 2nd grades) came up to me and said "Papa, the rocket launcher won't work". After taking a look at it, I told them that we had to fix it. So we went into the basement with all my parts and fixed it.
The second picture is what I found. I guess I should have paid more attention to their use of the transformer. I have never seen wheels melt like that. Sorry about the focus.