Got it, thanks for putting me right on that!
So in theory, heat from friction of the loco underside rubbing on the rails/sleepers could start a fire if there was a fuel leak. I'd have expected the fuel tanks to be a bit stronger though, then again there's a lot of weight and energy involved.
Normally they are pretty hefty chunks of metal - i had the displeasure (if thats the word) to see one of my favourate 47s cut up last year, and the time it took the mechanical grab to try and even pierce one of the fuel tanks was impressive. Even dropping it on the floor didnt significantly damage it. However - firstly, that was an Internal Tank - 47500 like most 47s has 3 - two inside and one underframe mounted tank, and with the derailment leading to all axles off track would have caused some siginificant underframe equipment damage anyway - as you say, with a derailment like this, there's going to be a substantial amount of weight / energy and friction involved, and with any standard rail debris lying around will only add to the damage.