The "subsurface lines" (Circle, District, Hammersmith and City and Metropolitan) are incredibly old as this article points out and failure prone. It is slowly b

The "subsurface lines" (Circle, District, Hammersmith and City and Metropolitan) are incredibly old as this article points out and failure prone. It is slowly being replaced with automatic train operation which should be a lot more reliable. Even on other automaticac train operation based signalled lines like central, northern, Victoria and jubilee you still get the occasional signal fault. Problem is if any of the hardware fails the whole system shuts down as a failsafe.