It was Winston Churchill who paved the way for the Brighton Belle. As Chancellor of the Exchequer, he announced in 1929 that the railway passenger duty on first and second class fares would be abolished - provided these sums were used by the railway companies to fund improvement and development schemes, a strategy which would reduce Britain's growing levels of unemployment.

In 1930, the Southern Railway Board decided to spend an estimated £2 million to electrify the lines to Brighton and Worthing from the existing limit of suburban operation at Coulsdon in Surrey. Work began in 1931.  In the interest of flexibility of train operation, it was decided to continue to use the existing 660 volt third rail DC system already in use on some 1,285 km (800 miles) of track. Colour light signalling would be introduced, enabling twenty two manual signal boxes to be closed. Power was taken from the national grid at three locations at 33,000 volts AC and distributed to eighteen sub-stations. Each sub-station contained a rectifier to convert the AC current to 660 volts DC.

It was all completed by October 1932 and the first public train ran on the 1st January 1933. This was a new type of electric railway offering fast non-stop express trains as well as intermediate stopping trains. While new but traditionally designed stock would be employed for stopping trains, it was decided that a higher specification for express trains would be needed.

The power cars would be of all steel construction, equipped with four English Electric 225 h.p. motors. These express units - assembled in sets of six cars - were the model for the Brighton Belle. The Southern Belle had been steam hauled; when the decision to electrify the line was taken, it was clear that its continued existence would depend upon the construction of an electric Pullman train.

Accordingly, fifteen of the express cars were built in three sets of five Pullman cars, to a similar basic pattern. Each set consisted of a motor third brake at either end. These cars had a large luggage compartment and forty eight third class seats and weighed 62 tons. They were each equipped with four 225 volt motors and control systems identical to those on the express units.

In the kitchens current was supplied at 110 volts from a dynamotor mounted on the under-frame. Lighting throughout was at 70 volts supplied by a 5kw motor generator. 600 volt heaters were provided between the seats and 600 volt immersion heaters provided hot water for the kitchens and lavatories. 

The three trailer cars consisted of two first class cars, each with kitchen and pantry and twenty seats, and one third parlour car with fifty six seats. Altogether, there were one hundred and ninety two seats in each five car set; they measured 102 m (335 ft) overall, and weighed 249 tons.

Unlike the express units, where only the motor thirds were all-steel, all five cars in each Belle set were of all-steel construction. Another unique feature was the use of cork as an insulator in the floors and with two thicknesses of an insulating wood material in the walls and ceiling. There was also an internal system of bells to enable the stewards to know exactly which passenger was calling for attention.  

Special attention was paid to ventilation. Fresh air was drawn past a thermostatically controlled heating chamber with 600 volt heaters and then through ducts a foot above floor level with small openings pointed downward.  Each car had a notice stating ‘This car is continuously ventilated with clean fresh air’.

Much attention was also paid to the upholstery, carpeting and wall decoration. Upholstery was supplied by J. Holdsworth  & Co. and T.F. Firth & Co. Carpets came from James Templeton & Co. and the rubber flooring for the third class cars was by the Ioco Rubber & Waterproofing Co. All the metal fittings such as lights, parcel racks and handles were by James Beresford & sons. Each car had a unique wood pattern of veneers and marquetry. All fifteen cars were built by the Metropolitan Cammell Carriage, Wagon & Finance Co. 

A more detailed factsheet on construction is available here.