On 5 October 1999 at 8.06 am, a light turbo commuter train travelling from London to Bedwyn collided with a high-speed train travelling in the opposite direction just three minutes after departing Paddington station. The collision occurred at a combined speed of 130 mph, killing both train drivers and 29 passengers and injuring over 400 passengers. Although there were many failures that contributed to the incident, one of the key issues explored in the subsequent inquiry was why the signaller involved took 18 seconds to respond to the alarm advising him of an unauthorised track occupation. This chapter presents this case study to determine whether the signaller's response time, criticised in the inquiry report and also highlighted in Lawton and Ward's analysis, was indeed inappropriate. For this purpose, the Critical Path Analysis approach was used to model the response times of the signaller.