Nobody East of Franklin and Attleboro will be going to Twin River when a Taunton Casino opens and vice versa.
The location is designed to divide the market almost exactly along state lines. Pretty much the only Massachusetts customers of RI and CT casinos based on relative driving times might be the ones between Sturbridge, Worcester and Franklin. That gives Taunton a reasonably good sized market with SE Mass and some extra customers in the Summer with rainy day Cape Cod tourism.
Sure that won't happen on day one with the regulars, but over about five years people won't be driving extra to get to Rhode Island.
Market share wise the interesting thing will be where the dividing line is between Wynn Everett and Taunton. People tend to think of Boston as a dividing point based on the risk of getting stuck in traffic on 93. My guess would be that Quincy and Milton would be about where the market is split 50-50 between Taunton and Everett, especially depending on the day of the week and time of day. Friday nights will go to Taunton and Saturday and Sunday will go for Everett.
Once Pawtucket commuter rail station opens it's a transfer to RIPTA Route 73 at the station and 18 minutes door-to-door to Twin River. Frequencies on all the RIPTA routes at Pawtucket terminal are scheduled to increase a lot when the CR station links to it. All TR needs to do is public-private themselves a transit deal like Wynn is doing with after-hours Orange service to add some 73 evening frequencies. That opens the floodgates to Massachusetts patronage. It'll be quicker and easier to get to Twin River from Mansfield than it is driving two towns over on Route 106 to Plainridge. The GATRA bus out of Attleboro can't compete either, since that takes twice as long to get to Plainville and simply doesn't traverse enough density to fill any usefully increased off-peak frequencies.
Twin River already advertises heavily in the Boston TV market. Once the accessibility catches up to them with RIDOT transit improvements currently in-design, it's game over for Plainridge.
Taunton doesn't become more accessible unless you do something about the godawful capacity pinch on Route 24 between 495 and 140. It's hell on earth at commute hours and on summer weekends--the only such spot on a highway that's otherwise free-and-clear south to Fall River and north to Stoughton--and MassDOT has no funds or plans to fix it. And commuter rail is not coming down the street...not even a Taunton Phase I at this point. I think their regional reach is going to be very limited with that part of the state being a population cavity outside of Taunton-proper and Fall River/New Bedford. They'd have to bank hard on patronage from FR/NB because too many points east on the South Shore and Cape have transit access dropping them off a South Station-Wynn shuttle away from ritzier environs, and easier highway access when Cape traffic's in offseason. Travel time really isn't that different despite short crow-flies distance unless you've got the luxury of a straight shot up 24 or 140 from Fall River/Newport or New Bedford.
Twin River already has pretty firm mindshare west of the Taunton River in places like Somerset, Swansea, Rehoboth, Seekonk that are Providence suburbs by any other name.
It really depends on how well the tribe executes on the resort, because they're going to be roughing for that first decade with all the FUBAR'd transportation right on their doorstep that's not going to get fixed any time soon. I'd predict a slow grower...slower than expected. But if they know what they're doing it can be a success.
Plainridge? I give them less than dozen years before they're out-of-business. Shit location, and Twin River's going to slay them when they gain tenfold better transit access in 5-8 years.
I just don't see anywhere the state can hang that SE license. Taunton, if done right, satisfies the market demand. Brockton is D.O.A. You simply can't plop a casino next door to the high school. It's already cultivated opposition on steroids, and the case for it was weak enough to begin with. All of the machinations with that license just reek of blocking games to keep Taunton at bay. Now that that's shot and Plainridge looks like it has no plausible path to success, nobody who knows what they're doing is going to stick their necks out for that region.
The only question is how long it takes for the Gaming Commission to realize it's only going to fill two licenses, and get down to business instead of chasing weak-to-nonexistent prospects with #3. It wasn't meant to be.