It was caused when a signalman's error accidentally diverted the Irish Mail express onto a dead end siding, where part of the train crashed through the buffers and into the River Anker.
At the end of the up (London bound) platform loop was a dead-end siding leading to a pumphouse by the River Anker with a small reservoir alongside it.
The signalman in the north signalbox however became confused after his watch had stopped, and was expecting a goods train, and set his points into the loop.
The driver, Samuel Taylor, and fireman, William Davis, on the engine, and one passenger, Rev.
[1] John R. Raynes wrote in 1921 that "it transpired that the poor signalman had been on duty 68 hours consecutively!".