The Second Corridor (previously Third Corridor) type of railway carriage was one of the standard mid-20th century designs, and was coded SK (previously TK) by the LNER and BR, and CF by the LMS. The layout of the coach was a number of compartments, all of which were second class (known as third class until 1956), linked by a side corridor.

The British Railways Mark 1 SK was the most numerous carriage design ever built in the United Kingdom. The original number series carried was 24000–26217. From 1983, those carriages in the 25xxx and 26xxx series were renumbered 18xxx and 19xxx.

There were two variants, those built for the Midland, Scottish, and Eastern / North Eastern regions had six seats per compartment, with fold-up arm-rests which folded into the seat-back, while those built for the Southern and Western regions, with their heavy commuter loadings into London, had eight seats in each compartment, and no arm-rests. Seating was of the interior sprung bench type. Over time, some SKs and BSKs were re-allocated from other regions to the Western, who tended to try to stitch the arm-rests in the up position.

In 1985, a number of Mark 2 First Corridor carriages were declassified to become SKs. They were renumbered from 13xxx to 19xxx, putting them after the end of the Mark 1 range (19452–19560).

No carriages of this type are still in daily use on the main line network, since open saloon carriages are now preferred by operating companies. However, some electric multiple units based on Mark 1 coaches have standard class compartments, such as . Many SKs still operate on preserved UK railway lines.

Designations for second corridor carriages are as follows:

LNER CodeDescriptionLMS CodeBSKBrake Second CorridorCHDMBSKDriving Motor Brake Second CorridorDMSKDriving Motor Second CorridorMBSKMotor Brake Second CorridorSKSecond CorridorCFTBSKTrailer Brake Second CorridorTSKTrailer Second Corridor

Livery In 1951 the initial livery for Mk1 coaches on British Railways was crimson lake and cream with black and gold lining, and all new Mk1 SK coaches were delivered in this livery up until 1956. In 1956 the standard livery changed to maroon with black and gold lining, except for the Southern region stock which adopted an unlined dark malachite green. Western Region adopted chocolate and cream on sufficient stock to operate its named trains such as the Cornish Riviera Express and the Torbay Express. In 1965 Manastral Blue & Grey livery was introduced as trailed on XP64 stock the year before and was the network standard for over 20 years. In the 1980s the NSE livery of white with blue at window level and red stripe under was introduced for stock in the south east of England.

Orders Lot No. Diagram Mark Built Builder Qty Fleet numbers Notes 30002 146 1 1951 Derby 180 24000–24179 BR1 bogies 30015 146 1 1951-2 Doncaster 40 24180–24219 BR1 bogies 30026 146 1 1951-2 York 82 24220–24301 BR1 bogies 30020 146 1 1952 Ashford 24 24302–24331 BR1 bogies 30007 146 1 1953 BRCW 36 24332–24396 BR1 bogies 30030 146 1 1953 Derby 40 24397–24436 BR1 bogies 30072 146 1 1953 Wolverton 10 24437–24446 BR1 bogies 30088 146 1 1954 Swindon 22 24447–24468 BR1 bogies 30070 146 1 1954 York 36 24469–24538 BR1 bogies 30068 147 1 1952 York 30 24539–24548 BR1 bogies 30070 146 1 1952 York 8 24549-24556 BR1 bogies 30070 146 1 1954 York 12 24557–24568 BR1 bogies 30070 146 1 1954-55 York 7 24569-24575 BR1 bogies 30057 146 1 1953 BRCW 100 24576–24675 BR1 bogies 30058 146 1 1953 Cravens 25 24676–24700 BR1 bogies 30059 147 1 1953 Cravens 20 24701–24720 BR1 bogies 30073 146 1 1953 Wolverton 25 24721–24745 BR1 bogies 30077 147 1 1954 Swindon 8 24746–24753 BR1 bogies 30078 146 1 1954 Swindon 42 24754–24795 BR1 bogies 30137 146 1 1954 BRCW 23 24796–24818 BR1 bogies 30153 146 1 1955-56 Derby 126 24819–24944 BR1 bogies 30154 146 1 1956 Derby 30 24945–24974 BR1 bogies 30208 146 1 1956 Derby 70 24975–25044 BR1 bogies 30155 147 1 1955-56 Wolverton 120 25045–25164 BR1 bogies 30230 147 1 1957 Metro Cammell 83 25165–25247 BR1 bogies 30231 146 1 1957-58 Metro Cammell 32 25248–25279 BR1 bogies 30494 150 1 1958 Metro Cammell 3 25280–25282 BR1 bogies 30349 146 1 1957 Wolverton 120 25283–25402 BR1 bogies 30350 146 1 1957 Wolverton 22 25403–25454 BR1 bogies 30356 148 1 1957 Metro Cammell 1 25455 BR1 bogies 30358 149 1 1957 GRCW 1 25456 BR1 bogies 30371 151 1 1957 Doncaster 1 25457 BR1 bogies 30374 146 1 1957 York 50 25458-25507 BR1 bogies 30737 152 XP64 1964 Derby 2 25508-25509 B4 bogies 30426 146 1 1957-58 Wolverton 146 25558-25703 BR1 bogies 30685 146 1 1961-62 Derby 202 25704-25905 CW bogies 30685 146 1 1962 Derby 67 25906-25972 BR1 bogies 30719 146 1 1962 Derby 87 25973-26059 CW bogies 30720 147 1 1962-63 Derby 78 26060-26137 CW bogies 30726 146 1 1962-63 York 80 26138-26217 CW bogies

Conversions In 1973-74 4 Second Corridor were converted to Class 309 TSK later become TSOL In 1976, 9 Second Corridor were converted to Stowage Vans 80431-80439 POT In 1977, 15 Second Corridor were converted to Sorting Vans 80381-80395 POS.

SK TSK Diagram 26024 71110 469 Class 309 26189 71108 469 Class 309 26196 71109 469 Class 309 26203 71107 469 Class 309

SK Type Year Diagram 25033 POS 80383 1977 726A/NS531 25045 POS 80387 1977 726A/NT531 25047 POS 80390 1977 726A/NT531 25056 POS 80395 1977 726A/NT531 25068 POT 80437 1976 727A/NT521 25071 POT 80432 1976 727A/NS521 25077 POT 80436 1976 727A/NS521 25078 POS 80384 1977 726A/NS531 25082 POS 80392 1977 726A/NS531 25083 POS 80385 1977 726A/NS531 25088 POS 80388 1977 726A/NS531 25089 POS 80391 1977 726A/NS531 25099 POS 80386 1977 726A/NS531 25103 POS 80389 1977 726A/NS531 25104 POT 80431 1976 727A/NS521 25109 POS 80382 1977 726A/NS531 25112 POS 80381 1977 726A/NS531 25117 POT 80435 1976 727A/NT521 25119 POS 80393 1977 726A/NT531 25127 POT 80439 1976 726A/NT521 25434 POT 80434 1976 726A/NT521 25139 POT 80438 1976 727A/NT521 25150 POT 80433 1976 727A/NT521 25156 POS 80394 1977 726A/NT531

Departmental use