My electric boxcab loco has operated for several years with a home-brew controller at first, then a locally sourced kit. The kit is labelled K275A and features multiple open frame SPDT (single pole double throw) relays to achieve the reverse. I was immediately suspicious of the relays as I had previously used a single DPDT relay, but the design included current sense and timed lock-out to ensure the relay contacts switched without current.
The loco recently suffered a severe derailment when it jumped over an obstacle, falling on its side and emitting flames from the PCB.
A workshop postmortem showed severe burning around the legs of a power mosfet, the mosfet obviously damaged, the gate resistor burned, the IC driver chip cracked open and the freewheeling diode short circuit.
I replaced all the obvious faulty devices, but found on test the diode still shorted. Eventually tracked down to a shorted relay contact.
It seems obvious that the physical shock of the derailment moved one of the relay contacts putting the battery directly across a switched-on mosfet causing all the damage.
I have now removed all the open frame relays and am in the process of installing my old DPDT relay, but retaining the current sense and timed lock-out.