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I used to regularly use Sheffield to Denby Dale tickets to commute Manchester to Sheffield as thetrainline thought they were valid that way. It was only ruined after one guard on TPE said something along the lines of 'we aren't thetrainline' and ' it doesn't matter what your itinerary says'
Off the top of my head, a few I can think of where they haven't been:
Hazel Grove to Buxton.
Edgeley Jn. to Chester via Altrincham.
Skipton to Carnforth/Carlisle.
Carlisle to Millom (if that's where they're banned from) via the Cumbrian Coast.
I don't think many have gone Edgeley Jn. to Stoke...
Indeed, I just wanted to highlight the relatively cheap cost of them. The figure banded about for ours is £400 over retail for the software, but ours have a lot more custom software and apps (in the region of 20+ off the top of my head) versus what a TOC would reasonably need.