Second Open or Standard Open (coded SO) is a British coach designation for open-saloon second class (later standard class) coaches with 2+1 abreast seating, usually seating 48 passengers.

The majority (95) were built using the same body shell as the Mark 1 SK and TSO.

[1] When the European railways agreed in 1956 to abolish third class, British Railways simply reclassified third-class vehicles as second class except for the special "Boat Seconds" which after some deliberation were finally reclassified as first class carriages in 1959.

There was also a one-off experimental 39-seat SO built by BRCW as part of the Mark 1 new prototype scheme.

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