I don't quite understand the accountants/auditors handle these short term settlements (ie the "assured" funding is less than the project duration/spend profile).
For instance the Tram, DLR & Picc line train projects are liable to go on a few years longer than the currently confirmed government support.
TFL seemed also to have maxed out its borrowing allowance many years ago & not sure it has cleared any of it yet.
Perhaps the only way forward is the lease type deal? (preferably direct with the manufacturer rather than a ROSCO)
If you read the TfL Board Programme and Investment Committee reports, you will see there are two figures, an authorised spend for the project, and a budgeted figure for this year.
So when the new trains are authorised, Government agrees the capital contribution, but usually in form X in financial year 24-25, Y in 25-26, Z in 26-27 etc
But what you don't see (because it is never in the public part of papers) is if new trains do not happen, then it will cost extra £A in 24-25, B in 25-26 etc to patch up and specially order parts to keep existing fleet still running. But this ongoing cost has to be found from the ongoing operational budget (instead of separate capital budget)
At some stage the whole life cost over next 35-40 years (nominal life of train) is cheaper if you start replacement now, rather than a date in the future. It is either blatantly obvious from numbers (in which case investment ought to happen quickly), or it is more marginal (and those in power tend to faff about and dither whilst trying to validate costings, often making troublesome changes to save money short term, and extending the process by years so doesn't happen at optimal financial time).