Even though the designations would appear to suggest that the SO was the standard type and the TSO a variant, in reality the TSO has been the default design of open coach on British Railways since the dawn of the Mark 1 era, built in large numbers, with comparatively few SO vehicles constructed, mainly for use as Third/Second/Standard class restaurant cars.

The designation TSO is also used for Trailer Standard Open in the system of British Rail coach type codes.

The first 20 Mark 1 TSOs (3700-19) did not have the centre door vestibule, so the 8 seating bays were spread equally along the saloon length.

The Mark 2b design saw the centre vestibule abolished, although the mid-coach divider was retained to still give two saloons of four bays each.

The basic layout of the Mark 2 TSO remained unaltered from the 2b to the 2f, although finish, materials, seating, and ventilation arrangements changed with each new build.

Prior to the introduction of the HSTs, the standard Anglo-Scottish express train on the East Coast Main Line would be formed of a Deltic locomotive and eight air-conditioned Mark 2 coaches.

[1] They are referred to as Open Standard, the original meaning of TSO having become obsolete as no Mark 3 SO vehicles were constructed.

The TSOE vehicles are specifically designed to be adjacent to the train locomotive and were built with a single corridor connection at one end only.