Noel,
We already have more than a few trains during rush hour, and continuous bidirectional service is the ultimate goal. When they get double-sided platforms in, Amtrak will let them run more trains, some passing sidings could increase the dispatching capacity more. There are two words to justify SLE: Q Bridge.
Why would a bus make better time? Busses hit stop lights, traffic jams, and a host of other wonderful things. With tourist-train track quality and freight diesels (i.e. not very good) RMNE hauls as it is, with some track improvements it would be fast. The roads around there are pretty slow. Plus, then it would have a 3-seat ride towards Stamford, instead of the a 2-seat ride. Probably neither would run north of Waterbury on weekends, buses because there would be no need, trains because RMNE would need the trackage on the weekends.