Flying Scot, you ask about the cause of this irritating event, I found with my Sumpter Valley loco that it had picked up a small piece of tree wood as it passed through Hedge End Station, pushed it along the tracks until it jammed in the next point causing a derailment, the leading chassis was totally off the track but the trailing drivers were still powering them both.
Now on my loco there is one traction tyre on the rear chassis which was trying to grip, whilst the other chassis was happy to carve up the railhead as they do, when very quickly restored to service it was running like a three legged camel because the rear chassis had jumped a single tooth, one of my members swiftly sorted the problem and the beasty was soon hauling it's 18 car logging train.
When I can find another chassis without a traction tyre I intend to retro fit that and hopefully have a more reliable locomotive.
John S.