The solution is simple: allow tickets issued between "London Thameslink" and Cambridge (& any other applicable locations) to be officially valid into Liverpool Street; there is already a precedent for this, in that tickets issued between "London Thameslink" and "Bedford Stations" are also valid into Euston.
Easy peasy.
The impliciation from that statement is that Cambridge to London Thameslink is valid via Tottenham Hale (but not via Liverpool Street). How would that work? The London Thameslink tickets are routed "not underground". So after staring at the "London rail and tube services" map for some time, the best option I can come up with to get from Tottenham Hale to the Thameslink core, with those limitations, is:
- Greater Anglia: Tottenham Hale to Hackney Downs
- walk: to Hackney Central
- Overground: to Highbury & Islington
- Great Northern: to Finsbury Park
- Thameslink: to Thameslink core stations
but that
feels dubious because you'd be going away from London on the Great Northern leg from Highbury & Islington to Finsbury Park.
The rules are as follows; there is no scope for feelings
1) Is it a through train? If so it's valid; if not, go to step 2
2) Is it the shortest route? If so it's valid; if not, go to step 3
3) Is it within 3 miles of the shortest route, and no negative easement preventing it? If so it's valid; if not, go to step 4
4) Is it a permitted route for any other reason, i.e. a mapped route, permitted by an easement etc. If so, it's valid, if not, it's not.
... but I don't believe that the presence of a gateline is a factor which can be used to determine the validity of a route.
Absolutely; all it means is that there is scope for staff to deny passengers the right to travel (either legitimately or, as I've experienced at London Overground gatelines, wrongly!)