Just to give you some idea of what you have to choose from, I've just pulled this book off my shelf. (Inside the front cover reads "Christopher, from Grandma and Grandad, Christmas 1964"), so it's exactly the era you're talking about.
The book runs to 300+ pages, and almost every single page features between one and about four different classes of locomotive or multiple-unit, ranging from steam locos organised by region (ex. GWR, SR, LMS, LNER, and BR) through diesel locos, diesel multiple units, electric locos, and electric multiple units, with the number (and name where appropriate) of every extant example of each class listed (and of course underlined by me when 'spotted').
(This page I grabbed from the net is from an earlier volume, my 1964 edition has only 8 A2s left)
I've just counted the number of different classes of steam loco in the ex. GWR section, and it runs to 45. Diesel locos 75 classes, including a good number of which there were only one or two examples, and to confuse things further some built by a company called English-Electric!
A new edition has been published every year, I think, from 1944 to the present, though recently under different names.
You'd probably be best to decide which region you want your railway to represent, to cut the selection down to an even vaguely manageable proportion.