Lots of good advice above. I run my railway to a timetable sequence, it can take anything from a week to several months to complete a "day" depending on how much it gets played with. Since I have been unable to control myself over the decades I have a selection of loco's and stock representing from the early/mid fifties to 2010 or so. My original intention was for my railway to be Southern Region based in the early sixties but it is currently in the mid/late fifties. At the start of each timetable day I decide which period I'm going to run that "day" and try to use only loco's and stock that would have been seen together in that period, these are roughly 56-58, 59-61, 61-64 although last year one "day" was mid 70s with blue diesels and DMUs. Between timetable "days" engineer's trains run and anything else that takes my fancy - anything goes! At the end of the day do whatever pleases you, it's your railway.