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Introducing a Transmission Based Train Control signalling system on London Underground's Northern Line facilitates an appreciable increase in train service frequencies, impacting the existing 630 V DC 'four rail' traction power system. Extensive operational and traction power simulation modelling was undertaken to investigate train voltage regulation especially under electrical outage and perturbed operating conditions. Traction power system upgrade options were developed comprising 'low loss' Composite Conductor Rail (CCR), increasing system voltage to 750 V, and parallel cabling. The upgrade options were evaluated in terms of equipment quantities, costs, installation programmes, and energy losses (including tunnel environmental impacts), and a single option taken forward for implementation.