Anyone who has caught a train will have noticed the distinctive way the rust coloured rails have a polished silver strip caused by wear by trains and grinding from maintenance crews.
At GZ there is a lot of rust, but a flash of silver will lead you to the cache.
There's been a station on this site since 1844 - originally called Abingdon Road, before being renamed Culham to avoid confusion when the station in Abingdon opened. Land alongside the station was commandeered by the navy in 1942 to create the most inland naval base, HMS Hornbill. The base included three runways. The base was later handed over to the UK Atomic Energy Authority in 1960 for the development of JET nuclear reactors.
Although few traces of the runways remain many of the hangars that surrounded the airfield still do, and the roads on the unused north part if the site can now be accessed by pedestrians and cyclists.
If you are interested in such things the older bridge here is one of the better examples of Brunel's railway bridges, constructed c1844 from Flemish red brick as the line was being being to the now demolished station in Grandpont, Oxford. Probably because it has been bypassed it's relatively unchanged from its original design, and is grade 2 listed along with the station buildings.
Although on a map this looks close to the road it is a bit of a clamber down bank to get to it. No pen provided, please use the cache fixing to tuck it back out of sight before you leave.