A First Open or FO, is a type of railway carriage used by British Rail and subsequent operators since privatisation.

Carriages from 3020 had a separate centre vestibule with doors and reduced-width bays, still seating 42 in 2+1 format.

Buck-eye couplers can be dropped and buffers extended to allow use of screw coupling fitted locomotives.

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.

The Southern Region introduced the yellow UIC cantrail stripe denoting first class in 1960 for boat train coaches.

The final development was the Mk2f being introduced in 1973-75 which the interior design from the prototype Mk3s with improved seating and air conditioning through body side heaters.

From 1979, seven First Open were used on the Glasgow to Edinburgh push pull service 11004–11010 with 11022 added later as 11004 was involved in the Polmont rail accident in 1984.