"77002" wrote

Was not the Hammersmith Branch a joint operations from the start?

The Hammersmith branch was built by the GWR and initially provided with
mixed gauge. It was opened on 13 June 1864 (i.e the year after the Met had
taken over operation of its own line). The GWR ran through, initially broad
gauge) trains to the City. Improved relations with the Met led to the
Hammersmith branch being vested jointly in the Met and GWR from 15 July
1867, after which the Met provided the basic service. When the line was
electrified in 1906 the stock was jointly owned.

Peter