[dredmorbius]

I just checked the clock times, and it's not possible to judge.

1953: no departure time, arrival is 13:05.

1983: departure at 14:00, arrival at 14:03. Fast trip! Or someone cheated setting up clock / departure shots.

2013: departure: 08:32, arrival 09:50.

The Brighton-London trip looks to be about 1:10 based on this site: http://traintimes.org.uk/?page=/service/timetable/lateroutbo...

Trip times here are 58m to 1h 20m. https://www.thetrainline.com/train-times/london-to-brighton

[j4mie]

The Brighton to London trip is just over 50 minutes on the fastest currently available route, which has two stops (East Croydon and Clapham Junction). The BBC article linked in another comment [1] mentions that the train used to make this video was a specially commissioned non-stop trip, so I expect it took a bit less than 50 minutes (assuming it was going at the same speed, of course).

[1] http://www.bbc.co.uk/news/uk-england-23853863