Dave, bust going up a rather large hill.
The crank had bust and been welded in the 1930's, we believe after a full astern + steam incident with a child. This broke the crank, a trunk guide and a cab upright (2 1/2 square ash) at least.
No other damage the second time. Though the engine did start to run backwards down the hill, got enough momentum to go over a chock. But it jackknifed the trailer and put a stop to things very quickly.
About a month earlier father experienced a very large noise like something breaking whilst driving down the road, but despite spending half an hour couldn't find any problems. Hindsight suggests that this was the crack getting bigger.
Also during its restoration, as a boy of about 15 I cleaned the crank in a bath of petrol, I swear blind I saw the crack, you know the petrol stayed wet in the crack despite the surface flashing dry. I got dad over and told him, but a lot of studying later the conclusion was that I was seeing things. Clearly I wasn't, but there you go.
Mammoth has got it in one for this break. Had been cracked many years clearly. Interestingly Marshalls had 4 designs of crank drawn up, we went for a modified Mark 4 with the manufacturer choosing his own radii (which were larger than drawn). A friend with another Marshall tractor suffered the same failure some 5 years later, in the same place.
I have to say the machining in the middle of a Foden crank looks very sharp also. But the roller eccentrics are ace, usually the limiting factor on a 5 tonner.
My mate is about to take delivery of a new crankshaft for his Foster Wellington tractor restoration. He has started with a bare but original boiler. He has had the cylinder made allready (a compound).