Press announcement tonight (16.04.24) that 10 Class 345s are being ordered for TfL Elizabeth line with work due to start in 2025, although no buisness plan has been submitted by TfL yet - this is not seen as a problem. To tide the plant over between completing what is on hand/stored to be finished (Class 720, 730, 701 cars) work will be transferred from other Alstom plants. It remains to be seen what exactly this is !! Presumably 10 additional 345s is going to mean 10 additional sidings somewhere to accomodate them (and additional drivers to drive them) - it dosen't end simply with the purchase of the trains !
I can't be bothered to trawl back through all the TfL Board minutes, and find the link, but 2 or 3 years ago wanted extra 5 for extending the Westbourne Park terminators to Old Oak Common.
Reading other recent news articles, seems Alstom wanted 10 trains minimum (of any type), so if it agreed that it is 10 extra 345s then it is 5 more than TfL asked for, so presumably will either have more spares or option to enhance a bit of service.
Regarding sidings, there will be 4 Elizabeth line platforms at Old Oak so at quiet times could use 1 or 2 of these to stable trains, similarly could use the 3 reversing sidings at Westbourne Park for stabling if not needed for reversals.
Until there is a formal announcement, I am not going to speculate further, because it might be a mix, eg 5 345s (if that was what TfL wanted) and something else (to meet Alstoms min 10 trains) eg more 730s (if this is a distress purchase who knows what face saving solution has been found, could be anything from extending electrification to Worcester as example). Harder politically to give them all to London.
One thing I am sure of is if talked had failed, neither Mark Harper or Alstom boss would have been posing for photos and posting it on X (twitter).