You might have expected that Birmingham services from Paddington would depart along the Relief Lines on the north side out to Old Oak, but they invariably seemed to cross over, even if coming from platforms in the centre of the station, to the Main Lines on the south side, then slow down to lurch across the quite tight crossovers at Old Oak to the Reliefs, and immediately turn off. You can see the Pullman do it in the video linked above.
The route to Banbury had various internal names, Joint Line was one of them, it was also known as the Bicester Cutoff. In the division of responsibilities the GWR did the signalling, so it looked no different to a normal GW line.
Unusual in WR days - the Paddington and the Birmingham suburban units rarely got onto each other's territory. Of course, once Marylebone took over the Baanbury line it was all suburban units, as that was all they had. Anything other than the Pressed Steel suburban units was also unusual at Paddington, except for a couuple of peak hour (only) services from Henley etc, where a few of the WR's large fleet of cross country, and later inter city, units were specially provided; otherwise these units roamed all over the region, except in the London Division.
