"Bruce" wrote

The Metropolitan Railway was originally built (from a junction with
the GWR at Paddington to Farringdon Street) to mixed standard and
broad gauge (7' 0¼") and was initially operated by the GWR using GWR
rolling stock. This is one reason why the Met has been able to
operate larger trains than standard.

Operation by the GWR with broad gauge stock lasted for 8 months. The GWR and
Met then fell out, and the GWR gave 9 days notice that they'd stop working
the line, expecting that the Met would give in and allow the GWR to take
them over. Instead the Met borrowed standard gauge stock from the Great
Northern and worked the line themselves. Mixed gauge lasted for some time
(Did the GWR use it for broad gauge goods?), and when the Widened Lines were
opened they too were provided with mixed gauge track, though it is uncertain
whether broad gauge trains ever used the Widened Lines.

Peter