British Rail Class 377

The Class 377 uses Dellner[3] couplers instead of the Tightlock type originally used on Southern's Class 375s. Southern's 375s were all reclassified to Class 377/3s upon conversion – these reclassified units can still be identified by their 3-car formation. Note that Southeastern's 375s (sub-classes 375/3, 375/6, and 375/7) were also later converted from Tightlock to Dellner couplers but were not reclassified; its sub-class 375/8 and 375/9 units were fitted with Dellner couplers, as built.[4]

All units can receive power via third-rail pick-up which provides 750 V DC. There are eight pick-up shoes per unit (twice the number of previous generation 4-car Electric multiple units), and this enables them to ride smoothly over most third-rail gaps. The units in the 377/2, 377/5 and 377/7 sub-classes are dual-voltage, and are fitted with a pantograph to pick up 25 kV AC from overhead lines. On these units (and on single-voltage sub-class 377/6), the shoe mechanism is air-operated so that when powered down, or working on AC overhead lines, they are raised out of the way. This is used on trains from Milton Keynes to East Croydon which use part of the West Coast Main Line between Milton Keynes and Willesden Junction, and then the West London Line towards Clapham Junction. These trains change to third-rail DC supply on a dual-voltage section of the West London line north of Shepherd's Bush.[5] Since March 2009, dual-voltage Class 377 units have also been operating some Thameslink Bedford to Brighton, Rochester and Ashford services. (See below).

Among the remaining units, the trailer coach in each unit has a recess in its roof where a pantograph could be fitted, to allow for future conversion to overhead AC power.

In April 2007, as part of the Route Utilisation Strategy for the Brighton Main Line, it was announced that Southern would procure an additional 48 Class 377 carriages to replace an identical number of Class 319 carriages (12 4-car units) due to be transferred to First Capital Connect.[6] Eleven further dual-voltage units were then added to the order, making a total of 23. They were ordered from Bombardier in March 2008 and were delivered in First's corporate colour scheme but with Southern spec interior (green seats).[7] The units were commissioned at Southern's Selhurst depot in Croydon before being transferred to First Capital Connect's Bedford Cauldwell depot. The first of these, unit 377501, was delivered to Cauldwell depot on 27 February 2009 after making its first appearance through the Thameslink Central London core. The 377/5s operate mainly on Bedford to Brighton services but in the peaks form part of FCC's and Southeastern's new joint service to places such as Rochester and Ashford.

Delays in the construction of the Class 377/5s for First Capital Connect saw the temporary transfer of eight of Southern's Class 377/2s to FCC to enable them to implement the planned timetable changes on 22 March 2009; the loss of these units until September 2009 was covered by the temporary cascade of a number of Class 350/1s to Southern from London Midland, which operated services between East Croydon and Milton Keynes.[8]

In September 2011, it was announced that Southern had begun the procurement of 130 vehicles, due to delays in the procurement of new Thameslink rolling stock that would prevent transfer of the 377/5s in time for the December 2013 timetable change.[9] The contract was awarded to Bombardier in December 2011.[10][11] The additional eight five-car units (from an option in the contract for 40 additional vehicles) are dual-voltage and known as Class 377/7.[12]

In December 2011, three Class 377/2 units were transferred from Southern to First Capital Connect (now Thameslink) to allow more 12-car services to be run.

During mid-2013, the first of the new Class 377/6s arrived from Derby for type testing, and since October 2013, these units have been used in passenger service, initially in peak-hour services on the Sutton and Mole Valley lines, Epsom Downs Branch, Tattenham Corner Line and the Caterham Line.

Southern Class 377/1 No. 377122 departing Warnham, with a service bound for Horsham

Southern Class 377/2 No. 377202 at Watford Junction

Southern Class 377/4 No. 377447 at Hassocks

The interior of Standard Class aboard a Southern Class 377/4

Southeastern Class 377/5 No. 377504 arriving at London Victoria

Southern Class 377/6 No. 377604 about to depart at London Victoria platform 9 with a Sutton service

Southeastern received 25 Class 377 units (the 23 Class 377/5s and 2 Class 377/1s) in recent years, as specified in Department for Transport documents, published in September 2013, relating to the new combined Thameslink, Southern and Great Northern Franchise.[13]

• Mainline and Redhill Routes: London to Brighton, Horsham, Reigate, Tonbridge, Eastbourne, Ore, Portsmouth, Southampton, Littlehampton and Bognor Regis
• East Coastway: Brighton to Eastbourne, Hastings, Ore and selected workings along the Seaford branch line
• West Coastway: Brighton to Portsmouth, Littlehampton, Bognor Regis and Southampton (selected workings)
• West London Route: East Croydon to Milton Keynes Central
• Oxted Line: London to East Grinstead
• Outer Suburban services: Central London to Tattenham Corner, Epsom, Horsham, Dorking, Guildford
• Inner Suburban services: Central London to Caterham, Sutton, Epsom Downs, West Croydon, Beckenham Junction via Crystal Palace (selected workings)

In December 2016 Southeastern received 8 units (377501-377508), transferred from Thameslink. In September 2017 these were joined by an additional 17 units (377509-523 & 377163/164).[14] These units will be serviced and maintained at Southern's Selhurst Depot.

Southeastern operates Class 377s on the following routes:

• Maidstone East Line: London Victoria/Blackfriars to Ashford International/Canterbury West via Maidstone East
• Chatham Main Line: London Victoria/Cannon Street to Ramsgate (selected workings)
• South Eastern Main Line: London Charing Cross to Ashford International/Folkestone Central (selected peak hour workings)

In 2016, 19 of the 23 Class 377/5 units were planned to be transferred to Great Northern, for use on non-stop London-Cambridge services. However, the 29 former Thameslink Class 387/1s were transferred, instead.[15] The Class 377/5s were later transferred to Southeastern, as part of their requirement for additional capacity.