Why not try a different company that isn't Network Rail, there's more to the railway than Network Rail.
If you want to get into signalling design, have a look into companies such as :-
Siemens Rail Automation (used to be Invensys Rail)
Balfour Beatty
Signalling solutions
Amey
Jacobs.
Thats what i can remember of the top of my head.
I myself have just finished a degree in Railway Engineering and hopefully going on to the Siemens Rail Automation graduate scheme.
If you have any questions I'll do my best to answer them.
Elliott