[1] The various boxes on this section of the railway used Sykes electromechanical "Lock and Block" interlocking, which used treadles to detect the presence of trains on the line rather than electrical track circuits, and prevented unsafe signal indications by mechanically locking the signal slides in the box, rather than interrupting an electrical circuit.
A feature of the Sykes system was that the various actions needed to set up a route for a train and clear the appropriate signals had to be completed in the correct order, otherwise the apparatus would lock up and require releasing.
In his evidence to the enquiry, he described this as a "treadle failure", but it was ruled that the lock-up had been due to his operation of two signals in the wrong order.
Between 0757 and 0804, no fewer than ten trains were due to pass Battersea Park, and Childs was working under considerable pressure.
Childs sent the bell signal, but (his fourth mistake), he pressed the wrong release button, leaving the apparatus locked.
Rather than attempting the release again, or contacting Battersea Pier by telephone to investigate the problem, Childs assumed the apparatus had failed, and decided to open the case and reset it manually.
This caused the block indicator at Pouparts Junction to return to "Line Clear", as it was not being held on by the switch-hook circuit.
[1] The enquiry recommended that the paragraph of the rules which permitted signalmen to manually reset the apparatus in an emergency should be deleted, so that any such maintenance tasks would only be performed by qualified linemen, and that the conversion of the signalling in the area from electromechanical interlocking to entirely electrical track circuiting should be expedited.