1007 Western Talisman, which had been manufactured in 1962 and was based at Laira Depot, where it had run an estimated 950,000 miles since its entry into service.

The door would lock in position whilst the train was running via carriage-locks that would be activated by a square-key, but the Class 52 had an additional feature in the form of a pear-shaped metal catch known as the "pear-drop".

At Paddington, Western Talisman was stored until the afternoon of 18 December, when the engine was worked light to Old Oak Common Depot for maintenance to be carried out to fix the problem.

[1] At some time between 06:30 and just after 07:00, Shift Supervisor L. Wiggins came to discuss matters and found that somebody had closed the battery box door.

Under the standard arrangements, Owen examined the engine before taking it to Paddington but failed to notice the unsecured door, primarily as his main responsibility when examining the locomotive was to check the train heaters, make sure the locomotive was set to run off its batteries, and to make sure no pipes or wires were still attached.

Later investigations examining the route that 1A82 took found that the projecting door had caused damage to lineside equipment and structures at Old Oak Common and at Acton Main Line, although none of the crew or passengers reported feeling anything unusual between these areas.

This impact was felt and heard by several witnesses, including Driver Owen and Secondman Woodnaugh, who both believed at the time that the engine had simply crossed over a bad rail joint and decided that they would report it at Reading.

[1] With the signals being clear for 1A82 and there being no sign of anything wrong, the train was still running at full speed as it approached Longfield Avenue, at which a crossover was placed to allow trains to cross from the Down Main Line to the Down Relief line in the area controlled by the Signalbox at Old Oak Common, which controlled the points by sending an electrical signal to the switch motor.

On the day of the accident, the crossover had not been used and was set for 1A82 to remain on the Down Main Line, but owing to the damage sustained by the door at Ealing Broadway, the door was in such a position that when the locomotive passed the motor, it hit the motor with some force, which forcibly changed the points and derailed the train, with the rear bogie of the engine taking the crossover whilst the front bogie remained on the main line.

The remaining coaches were all derailed, but remained in line and suffered only minor damage, with the rear of the train coming to a stand just past the damaged points motor, whilst lying between it was the battery box door that had been torn off by the impact with the motor.

His report considered that the primary fault of the accident was due to poor working procedures at Old Oak Common's maintenance area, with the fact that somebody (it was never determined who or when) closed the battery box door without securing it, or notifying the Electrical Engineer.

It was also noted that the primary error that caused the accident, Pitter's decision to secure the pear-drop, was caused by the fact that engineers at Plymouth had, without permission, made it so the pear-drop could be screwed into the raised position, which McNaughton believed was, although well-intentioned, unnecessary and could lead to instances such as this.