I should start out by saying that I generally oppose closing routes, but I actually think that this is quite a good exercise to ensure that we can make the case for rail, especially when comes to reopening proposals.
Apologies if this has been done before, but which rail line in the UK do people think has the best case for closure?
Perhaps you mean "which has the highest level of subsidy whilst offering the smallest community benefits that couldn't be delivered in another way"? Or "scrapping which line will save the largest amount of money (operating subsidy and renewals) over the next {n} years?"
In which case, the key will be how do you measure community benefit? For instance, in East Anglia, you could make the argument that scrapping the Reedham - Breydon Junction line via Berney Arms is unlikely to inconvenience many people (1500 passenger/annum at Berney Arms), but it isn't likely to save very much money. So it is a good candidate for closure or not?
Similarly, I suppose you could target the East Suffolk from Lowestoft south to Saxmundham (kept open for flask trains to Sizewell), but here there are (some) more people to inconvenience. Again, though I don't think that it would save that much money, and the (recently improved) 1tph service is showing signs of delivering (e.g., Beecles +20% year-on-year).
So I suppose it really matters how you measure "best to cut"; if it comes to this (which I fear it may, given the state of DfT's / NR's finances) then what we really need is subsidiarity so that regional government can decide either where the axe is to fall or whether they would like to make up the shortfall themselves.
