Ian@StEnochs
Western Thunderer
If all or part of the building (and site) still exists and remains in use then the plans will be held by Network Rail.
Otherwise, I understand all British Railways records went to the NRM/Science Museum and/or the National Archives at Kew, whether it includes buildings and estates I don't know.
Some British Rail records (1977 to 1998) were also passed to The Historic England Archive The Historic England Archive | Historic England.
However, I did come across this from the Greater London Industrial Archaeology Society who may have some information or provide a source as they probably details when excavating the site to know what they were dealing with. Greater London Industrial Archaeology Society
You would expect that to be the case but the reality may be very different.
When Troon station, B listed building, was destroyed by fire a couple of years ago it was found that Network Rail didn’t have any plans of the building. Luckily the G&SWR Association stepped in and loaned them a full set of drawings from their archives which enabled restoration to almost original condition. OK Troon is in Scotland but NR is a UK organisation.
Ian.