Colchester railway station is on the Great Eastern Main Line (GEML) in the East of England, and is the primary station serving the city of Colchester, Essex. Its three-letter station code is COL. It is 51 miles 52 chains (83.1 km) down the line from London Liverpool Street and on the GEML is situated between Marks Tey to the west and Manningtree to the east. Colchester is also the location of a major junction where the GEML links to the Sunshine Coast Line, which runs south to Clacton-on-Sea and, via a short branch, to Walton-on-the-Naze; services to and from Colchester Town also join the GEML at the Colchester junction. The junction is grade-separated so trains branching to and from Colchester Town or the Sunshine Coast Line do not cross the main line.

The Great Eastern Main Line is a 114.5-mile (184.3 km) major railway line on the British railway system which connects Liverpool Street station in central London with destinations in east London and the East of England, including Shenfield, Chelmsford, Colchester, Ipswich and Norwich. Its numerous branches also connect the main line to Southminster, Braintree, Sudbury, Harwich and a number of coastal towns including Southend-on-Sea, Clacton-on-Sea, Walton-on-the-Naze and Lowestoft.