All the 175s and the 3 car Chiltern 165s (assuming other stock turns up there) would be 103 cars, so would fit. And all under 1 ROSCO.Clicking "register your interest" brings up:
Great Western Railway (GWR) is seeking expressions of interest from suppliers for provision of existing fleets of between 10 and 120 Diesel Multiple Unit Vehicles (DMUs) in 2 or 3 car formations to supplement current services across the GWR network. These should be able to operate at the current sectional running times (SRTs), with similar seated and crush laden capacity to be introduced in increments up to and including the December 2027 timetable date , from the contract award date to the 29th May 2028.
Looks to be a gradual introduction, so Dec 27 would be the last unit to be accepted by GWR.Maybe I’m reading it wrong, but that sounds as if they’re planning on using some of them for all of 6 months (Dec 27 - May 28)
I reckon we'll see the bulk (probably 175s) sooner rather than later and further stock similar to what GWR already have (158s or 165s) later.