It seems as though this railway is all but abandoned. It appears that way when travelling through the site. If anyone knows any better, it would be good to hear from you. I have seen pictures on the internet, with notes about the railway, which say that it was last used in 2014.
I only came across the railway by accident as I explain in my blog:
I have been aware of the Longmoor Military Railway for some time now. I knew nothing of the Bicester Military Railway until my wife and I had part of a weekend in the area. We were travelling along the M40 and using Satnav and we noticed an interesting area off to the East of the motorway which was just visible on the Satnav. Our initial thought was that there might have been some opencast workings in the area.
Much of the site has been sold off. The northern half of Graven Hill Depot is now an estate of plots for self build homes! (https://www.gravenhill.co.uk