They were designed by a team headed by R.A. Riddles, who at the time was the Member of the Railway Executive responsible for Mechanical & Electrical Engineering
They were not without their technical problems at first, perhaps that could be expected. Allegedly some of the problems arose because Derby C&W dept. refused to consult with the Locomotive dept. during the construction.
A detailed look at the design principles and construction can be found in the Red Triangle Derby Lightweight section.
The body frame, underframe structure and panelling was of a light alloy (ICI "Kynal" wrought aluminium), designed as an integral unit in order to achieve lightness combined with strength. The structural underframe, coachwork and flooring members were formed from specially designed Kynal M39/2 extrusions, riveted together with light alloy rivets, and Kynal M39/2 sheets for panelling, Argon arc welded to form continuous sheets from door to door, which was riveted to the frame members. The whole of the inside structure and the underside of the floor was sprayed with blue asbestos to reduce condensation and noise, as well as for insulation. The floor was then covered with two thicknesses of flameproof hardboard and overlaid with linoleum.
BR's conventional bogie of the time was used, which was formed of mild steel, with the main frame members being riveted together and the individual members were partly fabricated. The bolsters were fitted with spring side control and side friction blocks. Roller bearing axleboxes were supplied by British Timken.
Each engine drove an exhauster which was used for the vacuum brakes, which used standard coach brake cylinders.
A dead mans device was built into the throttle, if this handle was released for more than 5 seconds all engines returned to idling, the gearboxes (or clutches on the West Riding sets) returned to neutral (which prevented the engine from stalling) and the brakes were automatically applied.
The 24v electrical system provided 60W lighting in the passenger saloons in standard coach fittings, and the drivers and guards lights were on different circuits. These and the control equipment were powered by alkaline batteries which were charged by dynamos, belt driven by the engines. Underfloor mounted oil burning combustion heaters provided thermostatically controlled warm air through ducting the full length of the vehicles at floor level on both sides. The heater control switches were located in different places depending on the type of vehicle. Driving vehicles had the switches in the cab. TBS's had them in the guards compartment, and on other centre cars above one of the doorways. During warm weather cold air could be supplied to the cars.
The power cars had AEC 150hp engines with a standard Wilson epicyclic gearbox. The control system for these differed from the West Riding batch, so they were designated "yellow diamond" (same as the Met-Camm Lightweights) as opposed to "red triangle" which the West Riding's were. One vehicle was trialled with automatic gear control system.
These cars had to be built with modified fittings to reduce the overall width to 8ft 11in, some 3in less than standard. This was because of the very limited clearance on the Maryport & Carlisle line. These cars had a strengthening bar on the inner windscreen, identifyable by the side located wipers.
Tubular steel framed seats were trimmed with maroon moquette in the third class and blue uncut in the first. The moquette in both classes was a kind of "flower" pattern, set out in narrow vertical lines. Vynide was used on head rolls and seat borders. First class saloons had a variety of seating arrangements, but second class was mainly one-direction style in a 2+3 arrangement.
Interior panelling was Vynide cloth covered flameproof hardboard, in various colour schemes. Luggage racks were provided along the complete length, formed from light alloy tubes of square section. All windows had pull down blinds. The toilet had primrose plastic panels and rubber floor tiles.