Tourist Second Open or Tourist Standard Open, abbreviated to TSO, is a type of British Railways coach. The designation "Tourist" was originally as opposed to a normal SO (Second Open) coach. Both types have the same number of seating bays per coach, but the TSO has four seats across, arranged 2+2 either side of a central aisle, while an SO has 3 seats across, arranged 2+1 with an offset aisle. Both offer the same legroom, but there is slightly less width per passenger in a TSO. 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. In 1987, British Rail changed the title to Tourist Standard Open, when "Second Class" became "Standard Class" across the network. The TSO designation remains in use for Mark 3A and Mark 4 Open Standard carriages., even though no SO coaches have been constructed since the Mark 2a build in the mid 1960s. The designation TSO is also used for Trailer Standard Open in the system of British Rail coach type codes. Whereas Tourist Standard Open applies only to loco-hauled stock, Trailer Standard Open is used in multiple-unit stock; in this case most have 2+2 seating but some have 3+2. Mark 1 The Mark 1 TSO contains eight seating bays, three transverse entrance vestibules, and two toilets arranged either side of a central corridor leading to the gangway at one end. The eight bays are distributed along the coach in two saloons of four bays either side of the almost-central door vestibule (this is slightly offset towards one end by the presence of the toilets at the other end), and the saloon nearest to the toilets is further divided into two saloons of two bays each by a transverse partition. The bay spacing is , this being the standard for Mark 1 Third / Second / Standard class, and identical to the compartment spacing in Mark 1 side corridor stock. 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. No Mark 1 TSOs are still in use