37254 on NR train at Portsmouth Harbour 14 2 2017

As Portsmouth Harbour station must be at least 50 miles from the nearest overhead line, I imagine the train carries out other inspection work as well.
For those not familiar with the power supply systems in the UK, electric trains and locomotives in the south of England pick up their supply from a third rail that runs alongside the track. Elsewhere overhead wires are used, on most electrified mainline routes. Metro and light-rail routes sometimes use three or four rail systems.
Portsmouth Harbour is about 20 yards beyond this locomotive, which is why this photo is in the UK Coastline and Beaches group.