Tim Dunn and Siddy Holloway get privileged access to the vast maintenance depot at Ruislip, at the end of the Central Line. It's the nerve-centre for all the engineering that takes place on the tube, from routine track repairs to emergency engineering. Tim gets a demonstration of how tracks are replaced, as a crane lifts huge pre-assembled sections of railway track into place. Siddy gets in the driving seat of a machine that moves ballast into place under sleepers to hold the track in position.At Warren Street, on the Northern and Victoria lines, Siddy explores the station's constant reinvention. It first opened in the early 1900s, was remodelled in the 1930s and then had a massive make-over in the 60s when the Victoria line arrived. There's plenty of evidence of its past - including exquisite original tiles in a former spiral stair shaft.At the London Transport Museum depot, Tim meets a poet who has had her work chosen to appear as part of the 'Poems on the Underground' series. She tells Tim about the idea behind the poem - remembering helping her mum hang out the laundry when she was a child.