Are the three videos running at the same speed? Or are they constantly adjusted so they keep up with each other so we can see more clearly any changes in landscape? I can actually believe that the journey time hasn't increased since the 1950s. However, possibly the clock at the end of the film is there to indicate the differing journey times? Anyway, interesting video. thanks for posting.
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