Tim Dunn and Siddy Holloway get incredible access to the maintenance depot at Northumberland Park in north London, responsible for looking after the entire fleet of Victoria Line trains. Tim meets the team checking all the carriages, and sees the incredible precision needed to finely shave the metal wheels to keep them running smoothly on the tracks. Siddy climbs the 1960s control tower and finds a vintage control panel still in working order, directing the points on the track to make sure each Victoria Line train coming into the depot goes to the right place.Tim and Siddy get to ride on the private service that links the depot with Seven Sisters station - alighting at its mysterious third platform.At Finsbury Park station on the Victoria and Piccadilly lines, Siddy does a night-time track walk to access the now-disused tunnel which was part of an early experiment to run large electric mainline trains on the tube.At the London Transport Museum in Covent Garden, Tim meets museum's CEO to hear about the importance of design across the tube network, and they look at the bespoke designs for seating fabric across different underground lines.