What is it you are looking for? Are you after routes or rolling stock? If its rolling stock then search the DLS for 'WD Naval', 'WD Conflat', 'WD Wagon', 'Gunpowder Van' there are also rail guns.
Try Google - British Military Railways - plenty of information there with both standard and narrow gauge listed Wikipedia will also assist.
You can search on Google Earth for the current layouts
Also search just north of Carlisle in Cumbria. Approx 12.5 miles NNW of Carlisle is another Military Railway and you can clearly see the rail layout but I cannot remember the name of this one.
I'm working on a donation based military route, though it's set in Germany. It even features a Gustav if I can ever get it out of sketchup and into Trainz.
I remember a layout plan for a military railway in Britain (training) in Railway Modeller some time ago - it might be worth doing a search to see if you can find out which issue. I have them dating back many years but to search manually would be quite a task! The name Liss springs to mind.
For a loco, a reskin of the J94 would be appropriate.
By their nature military railways are a bit sensitive. This one is a bit further afield http://en.wikipedia.org/wiki/Fort_Eustis_Military_Railroad and on the surface would make an ideal study, but one feels uneasy these days just calling it up on line or looking at Open Street Map let alone building a public released version in one of the sims.
