When it's a procurement of 250 cars, there's nothing that can be "fast-tracked". Procurements are enormously complex processes and typically have half-decade gestation periods at minimum, and with inflation and further industry consolidation afoot in the years since the CRRC deal was inked the bath the T would take on the new deal's unit price would be apocalyptic. In the end it's pretty much taking a similar amount of time for the same general product at enormously higher price.
It's still the best of a not-great set of options to continue nursing this order to completion any way they can. If CRRC is short on shells to continue assembly, then maybe what they can/should do is redouble their efforts to the warranty repairs on the cars that are in-service. Red's not running nearly as many sets of new cars as they have "active" and accepted on the property because of anomalous wear profiles on key systems on the CRRC cars that have been in-service the longest. It's piling them up in Cabot Yard waiting for the techs to have a look at them and how to correct the issues, and it's pretty much halted the retirements of the fast-failing 01500/01600 cars that need to be gone ASAP. While the warranty repair process is by its very nature overly cautious and doesn't necessarily foretell anything bad about the cars' overall reliability (i.e. it's a "normal" part of the teething process), CRRC is starting to fall well behind a pace that'll keep the in-service ranks stable and that's yet another problem that they're mismanaging. We'll at least be on an island of quasi-stability for the time being if they have enough in-service cars to retire the 01500/01600's...but we're not close to that yet the way they're stumbling on the warranty front.
Note: The wear/warranty issues are affecting Orange as well. It's just that with the order complete they're swimming in a surplus of cars until the signal mods are complete for enacting the peak headway improvements, so the effect of having so many "active" cars parked at Wellington for warranty mods isn't as noticeable to overall service reliability. But it has meant that repairing the 4 derailment-damaged out-of-service cars has been hopelessly far backburnered.