That's absolutely right.If the ticket is routed 'via London' then you consider separately the permitted routes from Wembley Central to London and from London to Carlisle when working out the routes you may take.
No easement required.I suspect there (but haven't checked) that there's an easement permitted a double back.
Taking an extreme example of this, consider Berkhamsted to Carluke, departing at 10am:
1002 Berkhamsted
1040 Euston
1110 Euston
1603 Glasgow C
1645 Glasgow C
1720 Carluke
You pass through your origin at 1127 and your destination at 1543.
It is impossible to make this journey without at least some doubling back, and the fastest journey includes a significant amount of repeated mileage.
The doubleback via London doesn't require an easement as the routeing guide includes mapped routes via London Group for this journey; the doubleback via Glasgow is covered by easement 700245 (" Customers travelling via Carstairs to Motherwell, Shieldmuir, Wishaw, Carluke or Lanark may double back between Glasgow Central and Motherwell. This easement applies in both directions. ")
Last edited: