There never was a Providence-Fall River-New Bedford rail route that hit all three cities. The Watuppa Branch between Fall River and New Bedford ended at Watuppa Mills, 120 feet higher in elevation than the Fall River Secondary along the river. A high-level bridge and tunnel under Fall River was proposed around 1910, primarily for the benefit of Cape Cod service.
The Watuppa Branch is largely intact (partially trail, partially active), but the Providence-Fall River route would be near-impossible to return to rail. You'd need to build a new tunnel under downtown Providence to replace the former viaduct, displace a very popular bike path with lots of grade crossings, reclaim a fragmented right-of-way with a lot of reuse, and build a new bridge into Fall River.
Any future transit is going to use I-195 at least between Providence and Fall River - whether that's express bus with some upgraded stops, light rail, or commuter rail.
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