When originally built in 1851 the Cambridge - Newmarket railway left Cambridge station immediately north of the platforms (which were shorter in those days) and just north of the cycle bridge turned 90 degrees ESE, crossing the other runnning lines and the goods yard, and headed through what is now Romsey Town, joining the current line SE of Brookfields. There was also a north facing chord which curved round behind Argyle Street and joined the main line close to Mill Road bridge. The arrangement at Cambridge was a logistical nightmare, and in 1896 the current alignment across Coldham's Common was built. The original line was disconnected at the Brookfields end and was used as sidings, gradually being lifted from the Brookflelds end.
http://www.disused-stations.org.uk/c/cambridge/index5.shtml gives a lot more history and some interesting photos. You can also see the course of the line slowly being swallowed up by the development of Romsey Town in the maps on http://www.oldmapsonline.org/en/Cambridge
This cache is a small sample tube located near where the line split into the north facing chord and the main line to the station,. If you look north, the lines would have crossed the road in front of you, the north chord curving away from you and following the ends of the Victorian buildings on Charles Street, and the main line curving slightly towards you and heading towards the station.