I'm interested to know if there are any constraints (platform lengths, stabling space etc.) that would prevent CrossCountry from running 8/9/10-car Voyager formations on their long distance services (Scotland to the South West, Manchester to Bristol/Bournemouth and Newcastle to Reading), assuming that the rolling stock was available. (I appreciate that it may well not be).
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I think I've read on here before that 10 car Voyager formations have to run with the back carriage locked out - 9 is the maximum, so XC try to avoid more than this. 8 and 5 carriages makes more sense than 9 and 4 anyway, given XC's very finite resources.
The SW-Scotland route is the only one with HSTs and the route with the most Voyager formations. Given engineering works can cause trains to get out of their usual paths at the weekend, I assume most trains on these services could be formed of double Voyagers if required, given that most routes operate a standard hourly pattern these days, so if it can be done one hour, it should be feasible in another hour.
I think one Bournemouth-Manchester service is a double Voyager so again presumably a number of them could be if required. However Reading and Birmingham New Street seem likely to be the bigger problems.
Exeter/Bristol - Manchester would potentially be hampered by New Street (where services not using the Camp Hill chord have to reverse), by the fact some services share platforms with Northern services at Manchester Piccadilly, and some services use platform 2 at Exeter St Davids which can only take 4 cars.
Southampton/Reading - Newcastle requires a reversal/parking at Reading and another reversal at New Street. Reversing longer services may add dwell time if it takes the driver longer to change ends. If there's a lack of space in several platforms at Newcastle for terminating services, longer services would eat up a lot of platform capacity and these seem the least likely services to run doubled up.
XC seem to prioritise the routes in that order so if they did have enough stock to run more doubled-up services, SW-Scotland seems most likely to benefit and Reading-Newcastle least likely in any case. That would roughly seem to fit in with the available infrastructure issues in any case.