Oldest and Newest (nearly)

As far as I am aware, Southern's class 313 EMUs is the oldest class of EMU and 313201 is the oldest EMU in service with a mainline passenger operator. On top of that, 313201 has recently been reliveried in a close approximation of it's original 1970s Blue and Grey livery, differing in the lack of the BR symbols and the sliding passenger doors being painted grey to comply with modern day accessibility regulations. The unit is in regular service on both East and West Coastway services radiating from Brighton.

As a contrast Thameslink are introducing Siemens class 700 EMUs and have completely replaced their class 319, 377 and 387 EMUs with these new trains. As part of the Thameslink programme Southern peak services to and from Littlehampton using class 377s have been replaced by Thameslink services using class 700s, in anticipation of these services being extended through the Thameslink core to Bedford. In preparation for this there have been additional crew training trips run by Thameslink between Preston Park and Littlehampton using class 700s, 700129 being in use today. 

This has meant the oldest EMU, and one of the newest EMUs running along the West Coastway during February in daylight hours, unfortunately not close enough for a side by side shot, but enabling the following composite image from the today 07/02/2018.

313201 at Elm Grove while working 2N17 13:03 Brighton to Portsmouth Harbour
700129 at Elm Grove while working 3H63 14:04 Littlehampton Shed to Preston Park Sidings