The Northern Line between Boscawen and Lebanon. While I agree with some of the above comments about how there was little business on this line at the end, I still think that this line should have been retained as a through route between Boston and Montreal. Sending freights all the way down the river and down the Fitchburg seems incredibly inefficient, and it seems like many miles could have been saved by using the Northern to it's full potential. Also, in the event of an accident or major disruption of service, having the Northern available would be huge. Also, even though the corridor would not be the best candidate for high speed conversion due to the curves and grades, it would still be a good option for passengers.
I'm old enough to have ridden the line back in the 1960s and photographed freights on it and I miss it too, but the factors you list weren't enough to support the cost of maintaining it when the pre-Guilford B&M was trying to cut expenses in the early 1980s and wouldn't be enough to pay to restore it today. There hasn't been any significant Montreal-Boston freight traffic since the 1970s, and when the B&M rerouted CP interchange traffic via East Deerfield it probably saved more on fuel running on the basically lat Conn River line than it lost on the slightly greater mileage. Today all that goes via CP's D&H line to interchange at Mohawk Yard in Schenectady, which is even longer but the CP wants to keep the traffic on home rails as long as possible. The Northern was famously used as a detour route after a Conn River derailment in the early 1980s, the last through trains on the line, but these days there is so little traffic on the Conn River (one NEC freight, one Amtrak train, irregular Pan Am operations) that maintaining a detour route for it is hardly justified. And as for passenger service, at the end there was just one daily round trip that consisted of one or two RDCs, and any restoration of Boston-Montreal passenger service these days will use the already passenger-quality B&A to Palmer and NEC north of there.
The Manchester-Portsmouth Branch between Manchester and Rockingham Junction. I think having a connection between the NH main line and the Western would be huge, even if it's only used for emergencies, or as needed. I don't see a line like this being used for passenger service, but as with the Northern, having it available for emergencies would be huge.
For what? There is hardly any freight business left in the Manchester/Concord area except for Bow coal, and a maintaining a detour route for a once-in-a-blue-moon Western Route blockage is hardly cost-justified.
In a world when even the Northeast Corridor has no viable detour routes, maintaining trackage as a contingency for what's left of rail freight business on the B&M these days isn't going to happen. As much as I hate to think about it, rail freight business in New England has dropped off so much since the 1960s and even more since the 1940s that a lot of fondly remembered lines simply no longer had a functional reason to exist.
