https://en.wikipedia.org/wiki/Longmoor_Military_Railway,
The Longmoor Military Railway (LMR) was a British military railway in Hampshire that was built by the Royal Engineers from 1903 to train soldiers on railway construction and operations. The railway ceased operation on 31 October 1969.
This was a training railway. It hasn't (thus far) had its own thread on the Coffee Shop, though a few mentions in passing in the past. It grew to connect a branch off the Alton line at Bordon to Portsmouth Direct Line at Liss, and (!) line a good model railway had a loop / circuit that trains could run round and round without having to run round at the terminals.
There seem to be lots of books on the "LMR" but few if any dedicates web sites, unless I have missed something.