Cambridge railway station is located one kilometre south of the city centre. From there you will need to take either a taxi or a bus which runs every ten minutes and stops at Drummer Street bus station (see above).
There is a frequent train service between Cambridge and London to either King's Cross Station or Liverpool Street Station. The trip takes about one hour. Trains leave roughly every half hour during the day. However, there is a five or six hour gap between about midnight and about 6 a.m. when no trains run between the two cities. Further details are available on the University's train times page.