The USA Trains magazine a while ago had a photograph of a heavy crane's tool van (actually a "Tool Car" there, of course), which has nosed down into a turntable pit, other end high in the air, with its crane most fortunately right behind, provided of course that the necessary tools could be extracted! None of the locomotives on the various turntable stub tracks were going anywhere until it was recovered. It was titled "Physician heal thyself", from the Biblical expression.
The majority of crane callouts were to simple derailments of wagons, or even trees across the line, for which the lesser capacity is quite sufficient. Major locomotive derailments are unusual in comparison. Even for the big cranes, much of their work was actually preplanned, for the civil engineering department, lifting in bridge decks etc.
Clearly there was some need of a crane at this location, whatever that was.
I have read a book by a railway breakdown crane operator, I think his "day job" was in Stratford works or somewhere similar... It seems that they were out with their crane to various East London marshalling yards on an almost daily basis as wagons were being derailed during shunting. So frequently in fact that it made me wonder whether perhaps the shunters got a kickback from the breakdown gang out of their callout bonuses!
Have just dug out my old (unofficial) note books I kept from back in the day, and at my old location we quite often assisted the passage of the Stewarts Lane 'crane & tool vans' on its way out or back home. Never took any pics of it though, so rely on Paul Bartlett's site for that.
It was normal for the crew to be those from the shed (or works) mechanical maintenance team, for they most likely included, on each shift, those who had done mechanics etc at technical college, and thus understood crane angles and such like. It was also usual for the overall crane rig to be on a siding at the shed, where a sudden requirement for a locomotive could be most readily obtained.
