The extension was supposed to have moving block signalling, designed by Westinghouse, in order to reach 36 trains per hour at peak times. As design of this overran, causing delays into 1999, this was postponed in favour of more traditional signalling. Twelve years later these features were completed, allowing for up to 33 trains per hour. It was built with a wider tunnel diameter of 4.35 m allowing a passenger walkway used for emergency purposes, unfortunately wider trains are not possible due to the narrower tunnel dimension northwards of Green Park.