https://www.londonreconnections.com/2024/longer-trains-for-crossrail/
Ideally, it's always better to allow for enough future clearance for longer trains.
https://en.wikipedia.org/wiki/Crossrail
It's even better to have longer stations or at least a roughed out provision established in the first place.
https://www.londonreconnections.com/2024/bonus-trains-for-crossrail-more-details/
https://en.wikipedia.org/wiki/Crossrail#Route
https://www.ianvisits.co.uk/articles/more-elizabeth-line-trains-arriving-in-summer-2026-73566
https://jfdatalinks.blogspot.com/search?q=The+Elizabeth+Line