Station modernisation and signalling improvements were incorporated into the scheme, but the Seacombe line was scheduled for closure. Electric power was taken at 11 kV 50 Hz 3 phase from the Clarence Dock power station; the cables were laid through the Mersey Tunnel. The traction supply to the trains was at 650 V DC by third rail. Goods traffic continued to be steam hauled. route miles of railway were electrified. It had been proposed to electrify at 1,500 v dc overhead, but this was technically impossible through the Mersey Tunnel, so the third rail system was adopted. The position of the third rail was the same as on the Watford New Line electrification, and required the Mersey Railway system to alter its third rail from 22 inches outside the running rail to 16 inches, and to fit duplicate collector shoes to its rolling stock. The new rolling stock was in three-car units of which there were 19, with steel bodies and open saloon accommodation. They had air-operated sliding doors, and automatic couplers. There was one motor coach in each three car unit, and it was equipped with four 135 hp motors. Additional power was considered to be required for the steep gradients in the Mersey Tunnel. Electrical equipment was supplied by British Thomson-Houston, and the vehicles were manufactured by Metropolitan Carriage and Wagon Company and the Birmingham Railway Carriage and Wagon Company. The units later became British Rail Class 503. Automatic notching was provided. When the trains were running in the Mersey Tunnel where there is a gradient of 1 in 27, provision was made to increase the notching point of the acceleration relay. This could be actuated by a sealed push-button in the driver's cab, for use in exceptional circumstances, such as a six-car train operating with one motor coach out of action. Tuplin wrote that the new vehicles did not have an on-board air compressor; at the end of the run: the motorman emerged from his compartment with a hose, lifted an iron cover plate from a hole in the station platform and connected the hose to an underground air-supply line. Incredible though it may seem, there was no compressor on the train, but reliance was placed on periodic charging of a reservoir which fed air to the Westinghouse brakes and also to the Westinghouse electro-pneumatic controllers for the traction motors. When a satisfactory charge had been obtained,