Alaric wrote:ahenry wrote: The definition of what is a "reasonable route" between two stations is easy here, but it can be really baffling.
Up to a point the National Rail enquiries site will do the leg work for you. You ask for trains between A and B, optionally via C. If it offers a single ticket, it's a valid route. If it says two tickets are needed, it's a case of "computer says no". That's where it gets arcane if you challenge that.
Kings Cross to Norwich via Cambridge is certainly a valid route and at some times of day, or starting from Kings Cross, quicker than alternative trains direct from Liverpool Street. 45 minutes non-stop to Cambridge makes a difference.
It is easy, in that if a ticketing web site will sell you a ticket for a journey, then it is allowed, even if it may be best to take a printout of the itinerary to convince a ticket collector. Understanding what is going on is not so simple. The complexity leads to anomalies, where tickets end up being much more expensive, or occasionally much cheaper than you would expect.
http://www.railforums.co.uk/showthread.php?t=70716 is a pretty clear description of the system, but it is a result of evolution, rather than intelligent design!
http://www.railforums.co.uk/showthread.php?t=139079 is a thread about reasonable routes between London and Norwich (GEML is the Great Eastern Main Line from Liverpool Street to Norwich, WAML is the West Anglia Main Line from Kings Cross to Cambridge).