It's then identifying the opportunities for five-car formations to come in handy, and the cost of the performance risks they bring in.
You might be able to save vehicle miles by splitting quieter services at Bristol, Reading, Newcastle etc., but you then have to weigh that up against the:
- Inevitable shortforms when the attachments and detachments fall apart when there are delays
- Cost of partial refunds for lack of seat reservations
- Lost catering revenue during overcrowding
- Cost of maintaining twice as many cabs
- Lower ratio of seats:carriages
- Cost of duplicate staffing in multiple portions
- etc.
I'm not saying it's not possible, but I just don't think there are enough services on XC where you could run single formations over enough of the route to make this worthwhile, especially with some aggressive advance pricing on quieter services and the release of supressed demand once using XC becomes tolerable.