I've always understood that the HSTs may have had nominal formations but in practice the cars were constantly switched around, and while the set numbers applied to the power cars the trailers were freely rotated, right from the start when the various different types were late in delivery.
An article in Modern Railways shortly after their introduction about a night shift at Bounds Green showed that they even had a dedicated Class 08 shunter at the depot for all the overnight switcharounds that took place, and were doing so on the night they visited (eg an AirCon module in a catering car failed and they had no spare unit, but did have a spare catering car, in itself something that doesn't go with fixed formations).
Interesting. Certainly on the WR in the early days there was a great deal of fluidity, partly due to the inability to form Sets with the booked vehicles due to late deliveries, partly due to various technical difficulties which emerged as the sets bedded into traffic, and also on the WR due to the changes in catering provision. That said, even when things had settled down, there were still individual power car and trailer swaps, but is evident from the records I have accumulated, that even in the midst of the difficulties and changes outlined, the WR stuck to the policy and
tried to keep sets together. A change did eventually come though in 1983/4 when the WR ceased efforts to maintain power cars to their allocated sets. I think this was due to the separation of power car and trailers maintenance cycles, and from 1984 onwards, trailer sets went to and from Works without their originally allocated power cars.
I have spoken to an Ex-BR Manager who was responsible for the implementation of the HST project on the ER. He told me that from day one, he directed maintenance and operational Controls to treat power cars as individual vehicles, and not as part of an allocated Set. Although my records are not as extensive for the ER, they do seem to show less consistency than that of the WR fleet.
It is clear that when the HST was conceived, the engineers believed/hoped that HST formations would remain fixed. In practice, they certainly were not permanent, but I do not think, as far as the WR was concerned, there was a complete abandonment of efforts to keep power cars together with their allocated trailers.
