Manual shooting and exposure.
If you have a camera with manual exposure. shoot with the background slightly over exposed, as most engines tend to be black or green. This will make the loco less dark.
Try to avoid any modern clutter, to keep that period charm. The only thing not period in the shot below is the black plastic bin bag. You can use good photo editing software to remove such anachronisms.
Shoot stationary trains for detail shots
For detail shots wait till the train is stationary and get as close as possible.
Having the driver in a shot adds interest and character. Drivers usually wear overalls and, if you are lucky a “period ” cap!
Shoot in winter
The East Lancs line, was shot below ,on a very cold January day. This made the steam much more obvious . The misty conditions were very period. This could be the 1950’s !
Shoot 3/4 shots
I usually shoot 3/4 shots that is with the loco 3/4 of the way around.
Use a zoom lens
A good zoom lens will bring the action to you. Try to avoid digital zoom, this will just drop the resolution. For flower beds and trolleys, crouch down so that you can get more distant buildings in the shot. This will also add drama.
Most lines have a guide to the best photography locations, check out the relevant web sites.
Make sure you have a timetable and find out which way the engine will be facing. On the Ribble steam railway the engine will be going backwards on the way out. This line does not have many good vantage points. One of the best is the level crossing close to the station and docks.