The Rail Accident Investigation Branch (RAIB) has released its report into signal T172 passed at danger at Purley station on 18 August 2006.
The full report is available here:

A summary of the key points from the report are: - On Friday 18 August 2006 a freight train travelling from Purley yard to Acton yard passed signal T172 at danger by 35 metres, following a shunting move at Purley station. The freight train was stopped by a Train Protection Warning System (TPWS) intervention. The driver immediately reset the equipment without speaking to the signaller, and continued his journey. The freight train was finally stopped by the driver at signal T160 at Purley Oaks station, which had been changed to show a red aspect. There were no injuries as a result of the incident.

The immediate cause of the Signal Passed at Danger (SPAD) was that the freight train driver incorrectly assumed that the adjacent signal, which was green, applied to him. The main causal factors of the SPAD were the driver positioning his locomotive incorrectly, and that no specific stop marker had been provided.

The immediate cause of the resetting of the TPWS incident was that the driver did not apply proper procedures after the intervention. The main causal factors were driver confusion, the incorrect assumption that an adjacent signal applied to his line, and the driver's lack of awareness and understanding of TPWS.