Blackfriars Crown Court was told that employee Alan Cooper was using a tower scaffold to clean the former lift shaft inside what used to be South Kentish Town tube station on 22 September 2014. He fell 9.2 m from the scaffold and was in hospital for ten days.
Inspectors from the Office of Rail and Road (ORR) investigated the accident and found the tower scaffold had been wrongly assembled and its stability had not been assessed. They said London Underground, a subsidiary of the capital's transport authority Transport for London, had not properly planned, managed and supervised the work.
The ORR also found evidence to suggest the company had procedures which could have prevented the accident, but did not follow them.
The rail regulator served two prohibition notices on London Underground, which it complied with. They related to the incorrectly assembled scaffold and safe access into the former lift shaft.
London Underground pleaded guilty at an earlier hearing at Blackfriars Magistrates' Court to breaching s 3(1) of the Health and Safety at Work Act. It was fined £500,000 and ordered to pay costs of £50,000.
Keith Atkinson, principal inspector of railways, said Cooper's injuries could have been fatal.
"London Underground has a good health and safety record," he added, "but this incident highlights why there can be no room for complacency."
South Kentish Town opened in 1907 and was closed 17 years later, in 1924, due to low passenger use.
