I just had a friend stay who returned today from Wilmslow, changing at Crewe, onto 1A38 which was a double Voyager from Chester (12:55 from Crewe), and he had reservations in coach B. I just tried making a booking for myself on 1A38 tomorrow and found that coaches C & J were the ones I wasn't able to reserve seats in. I would suggest to
Chucky to make a dummy booking for a train of interest and see what you can see for yourself, you don't need to get to the stage of paying for the ticket, simply ask for a reservation and then ask to change the seat reservation it gives you and it should be clear from there which coaches can't be reserved (I used the Avanti Web site for this).
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So my friend Bob’s experience plus my messing around implies coaches
C and
J, the middle coaches in each unit, do not have reservations.