I've not researched this properly yet, but I'm posting this as a starting point for discussion, and I'll probably revisit it later... I'd quite like a route with Request stops. For those not familiar with this these, they are stations where the train doesn't stop by default. If you're on the train, you tell the guard you want the train to stop; if you're on the platform, you stick your arm out like hailing a cab/bus. I like the idea of having to watch out for a person, or listening for a buzzer from the guard, would add a bit of uncertainty to the route. I think the North Warwickshire (or Shakespeare) line has 4 request stops: Danzey, The Lakes, Wooten Wawen and Wood End, so that would be a good choice. However, there are about 150 such stations in the UK, so there are plenty of other lines that would suffice.