I’ve written before, this segment — if electrified and served by right-sized EMUs — could easily handle new stations at Cavendish and Atwater, and become a de facto “surface metro” between Mtl-O and downtown. Like half an RER, upgrading service on exo1, 2, and 4 would make for decently short headways on the Westmount Sub, allowing show-up-and-go ridership.