This is the tunnel under Redstone Hill on the Redhill bypass line. It carries fast trains from Brighton and Gatwick to East Croydon and London, and vice versa. There are also quite good views of the hills to the North.
For the cache itself, find the numbers on the sign on the bridge. Let them be : a bcc de fg Cache is at N51 (f+g). (a*2)(a+b)(g). W000 cg.fc(a+f)
Haqre ebbgf npebff sebz 3eq pbapergr cbfg.