Refurbished 377415 Between Warblington and Chichester (Fast) 

Southern's Class 377 Elctrostar fleeet are currently in the process of refurbished, and this is one of the units: 377415.
This unit was working the 11:15 Southern service from Portsmouth & Southsea to London Victoria via Horsham & Gatwick Airport.
Southern's Class 377 Electrostars can be seen across the majority of the network, the only exception being between Hurst Green and Uckfield and the Marshlink Line (Ore to Ashford International). However the Class 377/2, /3, /6 & 7 are mainly dedicated to certain routes as follows
377/2 (dual voltage) - East Croydon/Clapham Junction to Milton Keynes Central (however they do appear on other Class 377 routes).
377/3 - Redhill - Tonbridge (I can't remember where else they operate).
377/6 - Metro services
377/7 (dual voltage) - Metro services (I'm not sure if they're also still used on Milton Keynes services).