"Recliner" wrote

I wonder whether the first Met
line trains 150 years ago were also GW broad gauge? I assume they were.

Yes. The Met was built as mixed gauge from Paddington (Bishop's Road) at
least to Farringdon and AFAIK to Moorgate, and was initially (Jan - Aug
1863) worked between Bishop's Road and Farringdon by the GWR using broad
gauge stock. The Met fell out with the GWR, who gave 9 days notice that they
would cease to work the line after 10 August 1863, but by then the
connection with the GNR at Kings Cross had been completed, so the Met began
operating the service themselves, using standard gauge stock obtained from
the GNR. It's not clear how much the broad gauge was used after this (GWR
meat trains to Smithfield, perhaps), though when the Widened Lines were
built in 1866, mixed gauge was provided, though it is not known if GWR broad
gauge trains ever used the Widened Lines.

Peter