The trains on the Central Line are generally regarded as lemons, and recently they've been suffering serious motor problems* causing service disruption. London Underground appear to be taking drastic action to get it solved, as they propose spending £90-130m to replace the entire drive system of every train:
Replacement of existing DC traction system by new AC traction system on approximately 85 trains each of 8 cars. Requirement includes replacement of 4 electric traction motors and related equipment on each car.Almost all new electric trains are built with AC motors - including the Jubilee and Northern Line fleets - as they're simpler, smaller and more efficient. Note that "AC" refers only to the design of the motors and the way they're controlled, so the fourth rail DC supply system on the line will not need to be replaced.
It would previously have been Metronet's responsibility to solve this problem. You may like to ponder whether they'd been putting off fixing it properly.
(* if you don't trust that rant, the problem is also mentioned briefly on page 33 of the Feb 2008 board papers)