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.

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Quick Facts British Rail Second Corridor, In service ...

British Rail Second Corridor
A Mk1 Second Corridor (SK) coach at the North Norfolk Railway in 2007
In service	1951–present
Manufacturer	BR Ashford/Easteigh, Derby (C&W), Doncaster, Swindon, Wolverton, York, BRCW, Cravens, GRCW, & Metro Cammell
Family name	British Railways Mark 1 and Mark 2A, 2B and 2C
Constructed	Mk1 1951–1963 XP64 1964 Mk2a 1968 Mk2b 1969 Mk2c 1969-70
Number built	2168
Fleet numbers	Mk1 24000–25507, 25510–26217 XP64 25508-25509 Mk2a 19452-19473 Mk2b 19478-19513 Mk2c 19515-19560
Capacity	Mk1 48 with armrests 64 without Mk2a/b/c 42
Operators	British Rail
Specifications
Car length	64 ft 6 in (19.66 m)
Width	9 ft 3 in (2.82 m)
Height	12 ft 9+1⁄2 in (3.90 m)
Doors	Manual slam doors, three on each side
Maximum speed	BR1: 90 mph (145 km/h) B4/CW: 100 mph (161 km/h)
Weight	32.5 to 36.5 long tons (33.0 to 37.1 t; 36.4 to 40.9 short tons)
HVAC	Steam or electric heat, or both
Bogies	BR1, Commonwealth or B4
Braking system(s)	Vacuum, air, or both
Coupling system	Drop-head knuckle coupler on draw-hook (dual)
Track gauge	4 ft 8+1⁄2 in (1,435 mm) standard gauge

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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.^{[clarification needed]}

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 Class 438. Many SKs still operate on preserved UK railway lines.

Designations for second corridor carriages are as follows:

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.

• 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.

A second corridor coach with a guard's brake compartment is designated BSK. The Mark 1 BSKs had 4 compartments, built as two variants with or without arm-rests depending on region, as with the SKs.

• Corridor coach