I spent about 5 hours watching the rerailing operation this evening and chatting to some of the staff.
All references to left, right, leading and trailing below are relative to the train as it was when departing Paddington. Power car refers to the one that derailed unless clearly noted otherwise.
When it came to rest the leading bogie of the power car had pivoted to the left, such that the leading left wheel was derailed to the six foot side of the left rail and the leading right rail was to the four-foot side of the right rail. All other wheels on the left side of the power car did not derail. The rear right wheel of the trailing bogie was derailed to the four-foot side of the right rail. I was only able to observe from the platform 1 side so I could not see the position of the other two wheels, but based on observations of the actions of the rerailing crew, the trailing right wheel of the leading bogie was also derailed, almost certainly to the four foot. Based on the actions of the staff and photos I've seen from right after the derailment occurred, I believe it likely that the leading right wheel of the trailing bogie was also derailed. If it did derail it did so to the four foot.
The immediate cause of the derailment of the trailing three axles (assuming all derailed) was gauge spread. It is likely that this is also responsible for the derailment of the first axle, but it is not impossible for the gauge spread to be the consequence of the leading axle derailing. I do not know in which order the axles derailed.
The track here is supported by chairs on longitudinal timber bearers. Approximately where (or very shortly ahead of where) the inner (leading on departure) bogie of the power car came to a stand on arrival on the previous journey, the timber bearer supporting the right (platform side) rail gave way. It is believed (but not proven) that this was the or at least a causal factor. All the investigation activity was focused on the track, and almost all around the point of the bearer damage.
The unit travelled a distance of approximately 50 chairs from start to final stop. At least two, possibly four wheels ran on top of the clips securing the right hand rail in the chairs and/or the bolts securing the chairs to the bearers. From a point approximately 10 chairs after the point of the obvious bearer collapse to where the unit came to a stand most (but not all) clips were missing and some of the bolts were clearly damaged. There was no obvious track damage at all between the point of the bearer damage and the buffers, so it seems very unlikely any wheels ran in the four foot before this point. A section at least of platform 2 will be relaid. Preparation for this was starting when I left around 11pm, but I don't know how long it will take. One of the rerailing crew did not think it would be completed overnight.
The break pipe was undamaged and did not separate. The rear power car was detached and the rest of the train moved about half a coach length forwards for ease of access to the derailed leading bogie by the rerailing crew, but once rerailed the train was connected up again and appeared to pass a break test. The power car did not suffer any significant damage that was apparent to me or to those who inspected it, and after rerailing it was dragged to the depot (presumably Old Oak Common) at slow speed where it would undergo a full exam. No wheel skates were required.
It was reported that when the unit derailed it did so with a very loud bang. I was not there to hear it but my supposition is that this was the sound of one of more wheels dropping off the rail into the four foot. It was also reported that the guard gave the signal to stop but it was sufficiently loud that the driver /might/ have been able to hear it. Whether they did or not is unknown to me.
I took lots of photographs but have not had a chance to download them yet.
