I love the idea of the Trumpton - Camberwick Green CrossRail (maybe an idea for Taunton - Norton Fitzwarren)
Twas indeed the Post Office Railway (or Mail Rail as it was latterly known). A small section under the Mount Pleasant Sorting Office reopened as a visitor attraction on 04/09/17 and well worth a visit. You don't get to travel in original stock, but completely new stock built for the purpose. There is still plenty of original rolling stock dotted about.
