How the accident happened

The accident at Ladbroke Grove in west London, near Paddington station, occurred at 08.11 BST on Tuesday, 5 October.

The Great Western Train (red on the diagram above) was the 06.03 from Cheltenham to Paddington.

It was travelling eastwards towards Paddington on what is technically known as the "up main line" (ie left to right on line 2, above). It was following a previous train along the same route at a safe distance and was receiving green signals.

The Thames Train service (blue) was the 08.06 from Paddington to Bedwyn (heading left on the diagram in the general direction of Reading).

It had started from Paddington on line 3, then crossed to line 4 before returning to line 3 on the approach to signal SN109.

At this point, line 3 stops and there are two routes which the Thames train could have taken.

It could either have dropped down briefly onto line 2 and then down again via connecting rails onto line 1, the "down main line", or it could have headed onto line 4, the "down relief line".

SN109 was red - as were all other five signals on that gantry.

However, the Thames train continued forwards on line 3 and past the connection to line 2, then past the connection to line 4 - either of which would have been possible routes to its ultimate destination.

At this point, a signaller at the Slough Control Centre who had been observing the progress of the trains on a VDU realised that the Thames train was heading into the path of the onrushing Great Western high speed service.

He changed signal SN120 to red in an attempt to halt the express, but there was not enough time. Almost immediately, the two trains collided with catastrophic effect.