Ex-Metropolitan Railway locomotive 12 is one of the 'lucky survivors' from the end of the locomotive haulage era. Seeking to find alternatives to dust-producing cast iron brake shoes London Transport engineers looked for a test vehicle for carrying out controlled tests on new brake pad materials. Former Metropolitan locomotive 12 was soon to become redundant from shunting newly delivered multiple unit cars and offered a self-contained and quite heavy traction unit. Modified to reduce the number of braked axles for stress-testing purposes the locomotive become the engineers' ideal solution!
Still working into the 1980s, 12 Sarah Siddons was requested for railtour service and became recognised as a valuable operational heritage locomotive. Placed under the control of the London Transport Museum and overhauled at Acton works 12 Sarah Siddons was rewired to more modern standards and adapted to allow operation on the BR Southern region 750vDC third rail system (LT uses a 630vDC four-rail system), allowing the locomotive to represent London Transport at depot open days and haul railtours on the third rail network.