BR's Swindon Works were chosen to design and build express units for the ex-North British Railway Edinburgh Waverley to Glasgow Queen Street route.

The cars were of integral steel construction using the bodywork as a stressed member, as opposed to the contemporary Mk1 coach design which relied on the strength of a trussed underframe.

[1] Each of the two power cars was fitted with two AEC 150 hp underfloor engines with mechanical transmission giving a maximum speed of 70 mph.

[1] Neither end featured any destination blinds or marker lights, only a box holding a stencil indicating the train classification (A, B or C; later changed to 1, 2 or 3).

The first series (79xxx numbered) were introduced to Glasgow Queen St to Edinburgh Waverley services, including those operating via Falkirk Grahamston, in January 1957; the units were allocated to Leith Central depot following its conversion from the closed railway station in 1956.

The Ayrshire Coast stock fared rather better, surviving almost intact until mass withdrawals in the early 1980s, the last two units being withdrawn in January 1983.

Although used on various other routes out of Glasgow in their final years, these 126s remained closely identified with the Ayr line until displacement by a mixture of loco-hauled trains and other DMUs.

The three Ayrshire vehicles have been completely overhauled inside and out with the assistance of the Heritage Lottery Fund and are currently in early DMU Green livery.