800 mile triangle is about 250-300 miles per side. You wouldn't need TGV level speed (180mph) for that to be effective. You could run trains at an average of 100-125mph, which is not much above current levels in the northeast (BOS-WAS expresses average about 80, on a route with little significant capital investment since 1939) and still have an extremely competitive service, at much much lower constructions costs.

It would still be a big project...but the economics would work much better if you aim for 2-3hr journey times instead of 1hr30.