We must remember ROSCOs do more than finance stock. Especially mid life upgrades. They did a lot of work financing disabled people mods to rail vehicles.
Do the DfT have that expertise?
The DfT, if I am to get ahead of myself on imagining a sprawling bureaucracy, would have a department of rolling stock management within the DfT where mid life upgrades would be handled, financed, arranged with e.g. Brodies, Alstom Longsight etc, but the UK is not into big govt culturally so again, just speculation.
