Railway signal
When the line was opened from Finsbury Park to Hammersmith, its signalling system was identical to the Bakerloo and District lines. Small cabins at each location of crossover installed controlled the signals there. Single lamps displayed track clearance in the form of green or red, with variations of yellow at difficult to spot locations. These equipment were supplied by Westinghouse, operated using compressed air. The exception was between West Kensington and Hammersmith, where it was controlled by District Railway signal boxes, and had semaphore signals instead. The Piccadilly line extensions resulted in resignalling on tracks west of Barons Court. Signal cabins were adjusted and new ones were added at Hammersmith, Acton Town and Northfields. A mixture of semaphore and coloured light signals were used on the four-track section. Signalling were redone on the new Uxbridge branch.