Class 508

The Class 508, part of the 1972 Standard PEP design like the Class 313 and 507, was built for the Merseyside DC third rail network to replace older stock. However as there was a need for new stock on Southern Region they served there initially until the new Class 455 could be built [1]. The Class 508s were built as 4-car sets with an extra trailer instead of 3-car sets as originally intended but after 4 years when they were finally transferred to Merseyside the extra trailer was removed and became part of the Class 455/7s [2].

Information
Number built: 172 cars (as built 4-car sets now 3-car sets)
Built: 1979-80
Builder: BREL York
Engine: 8 GEC G380AZ traction motors per unit (750v DC third rail)
Power: 880 hp (657 kW)
Formation: As built: Driving Motor Standard Open (DMSO)+
Trailer Standard Open (TSO)+TSO+Battery Driving Motor Standard Open (BDMSO)
Now: DMSO+TSO+BDMSO

Not all sets went North, some remained in the South East as Class 508/2 though none of these are now in service. In the early 2000s 3 sets worked alongside the Class 313 on the Euston-Watford route [3] as Class 508/3 but nowadays the only 508s in service are the 27 Class 507/1s sets working on Merseyside along with the Class 507. They remain among the oldest stock still in service though Merseyrail plan to replace them with new stock within the next few years [4].
Merseyrail 508 115 at Rock Ferry

Merseyrail 508 114 at Hamilton Square

Merseyrail 508 128 at Rock Ferry

[1] Brian Haresnape & Alec Swain, Third Rail DC Electric Multiple Units (Ian Allan, 1989) p. 74
[2] Bruce Oliver, Southern EMUs Before Privatisation (Ian Allan, 2010) p. 86
[3] Colin J Marsden, DMU and EMU Recognition Guide (Ian Allan, 2013) p. 389
[4] "Five shortlisted for new Merseyrail trains", Today's Railways UK No. 171 (March 2016)