The maximum length of an IET is 12 vehicle, a fundamental limit set in the requirement whatever the unit lengths might have been.

As I understand it, this is roughly how the allocation feature in RTT works. RTT gets train formation data from TOCs by agreement, not via any "official" NR data feed. The TOCs must get the information off their own train planning systems. If a train contains two units then two allocation data items are sent. If an extra unit is coupled to lengthen a train already notified then a second allocation item is sent.

If one unit replaces another there ought to be an allocation deletion item as well as a new allocation item. But if the deletion item is missing, RTT has no way of knowing this is not two units being coupled. No doubt a check against the coupled length limits could be added, but it hasn't been (yet).

 

Platform 5 at Paddington, from where it started, won't accommodate more than 10 carriages? 

 
I was looking at RTT to see which trains sped through Chippenham yesterday evening while I was waiting for my Melksham service, and spotted this:
https://www.realtimetrains.co.uk/service/gb-nr:W34637/2026-07-30

Do 14 coach IETs really exist? I didn't notice that it was any longer than usual, but it was motoring.