The cars were of steel construction and featured Pullman gangways and buckeye couplings. Each power car was fitted with two AEC 150h.p. underfloor engines with mechanical transmission giving a maximum speed of 70m.p.h. Initially termed "Inter-urban", this was soon changed to "Inter-City" long before that branding was used for main line express services.

These trains had two front-end designs: either a full-width cab or a half-cab with central gangway connection. The latter "intermediate" driving car allowed through access within a six-car set. 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). Power car seating was of the "open" saloon arrangement with compartment seating in the trailers. A unique multiple-working control system (coded White Circle) was employed. Unlike other classes, each power car produced its own control air supply and was thus incompatible with any other.

The second batch was closely based on the 1956 stock, one big improvement was that the guard's van was moved to the rear of the coach giving passengers a forward view. Four-character headcode displays were fitted; the intermediate power cars having a two-character box either side of the gangway. Destination blinds and marker lights were not incorporated.

The first six three-car sets worked their first three years on Western Region on Birmingham-South Wales services. There was criticism of the lack of a forward view for passengers and also of the plain "utility" appearance of the non-gangwayed cab front. The usual E. & G. formation was a six-car set with two trailers together in the centre of the train. The class usually stuck to this route but could appear on other services from time to time.

Almost all 126s were delivered in Multiple Unit Green with cream lining. The two exceptions (the Ayrshire Buffets) appeared in the darker Brunswick Green