I could be wrong with this guess, but I suspect this isn't a big end or indeed a motion failure. It is my feeling that a motion failure that then hit the lagging would have caused more twisting and crushing of the lagging.

I believe there is a mud hole door on the underside of the boiler at about this point and I suspect that this door has failed. The resulting sudden gush of steam and water could easilly be expected to cause just such a result. I also note the water lying on the running boards at this point as further indications of this as the cause.