Marks Tey is the junction between the Great Eastern Main Line between Norwich and London and the branch to Sudbury. As I was catching a train myself, I didn't get much of a chance to explore.

Platform 1:

Marks Tey Platform 1

The ticket office and bridge on platform 1:

Marks Tey offices

The building on platform 3, where branch line trains depart:

Marks Tey Platform 3 building