I agree with the comments above that the best stuff was shot on 35mm film. The British Transport Films (BTF) are very good, and are currently being re-released in sets of 2 DVDs by the British Film Institute (BFI) at intervals. Most have been commented on by myself and others in other threads in this part of the forum. In a number of films the railway is peripheral to the main action - almost taken for granted that that was how people travelled in the 1950s in particular. BTF films are limited to the BR period 1949-1986 when the unit was closed down.
Video125 have collections of 35mm film taken in the 1920s, 30s, 40s, 50s and 60s of steam - mostly for use in feature films but which was never used. Includes 'out-takes' from "The Titfield Thunderbolt" and "The Ladykillers". Has the bonus of being introduced by the late John Huntley, a great transport film enthusiast. Also a collection of items from 'Pathe Pictorial' and other newsreels.
Do bear in mind that early film taken by amateurs was mostly 8mm, and both this and even 16mm can sometimes not transfer to the TV screen well, particularly if poorly exposed or from a hand-held camera. Likewise some of the early video footage can suffer from mis-exposure. But we can't complain too much - at least something was recorded!
The Signal Box at Anstey, Leicester, has a huge catalogue of railway-related DVDs (
www.signal-box.com) - I have found their service quick and reliable. Many shops at preserved railways also carry considerable stocks of railway DVDs and are worth visiting or looking up on the internet.
I would avoid in general the cheap DVDs available from High Street outlets. These can be very poorly compiled and edited compared to the specialist companies.
Good viewing,
John Webb