It’s the (
infamous) RGU rail grinder. This often gets outstabled at Moorgate before/during/after a programme of rail grinding works on the sub-surface railway. It can only transit at low speeds, and also is not fitted with the new CBTC signalling now in use on the majority of the SSR. As a result it takes a long time to transit to/from its base at Ruislip depot, limiting the amount of useful railgrinding work that can be done. By outstanding at Moorgate it means that there is less time to travel to site, and more grinding work can be achieved.
Rail grinding is a maintenance process to essentially reprofile the running rails, leading to smoother rides and less noise.