So, I've been searching for a replacement. I've been to the diner just down the street from Rein's, which is open later than Rein's. The food's a little better, too, but it's still too close to Boston (and slightly further from the highway). There is a wonderful sushi place (variously called Hana or Hama, depending on which sign and/or phone book listing you encounter) in Hamden, but they're far from the highway, they aren't open for lunch, and sushi tends to be slower and more expensive than I'm looking for in road food.
Meanwhile, I've been getting to know the back roads of New England by going on a lot of road rallies. As part of this experience, I've tried a bunch of restaurants, some of which have been outstanding.
It was only on my way to New York a week and a half ago that I connected these two activities. We stopped on the way down at the Cromwell Diner, on Berlin Road (route 372) just west of exit 21 on I-91 near Middletown. We stopped there on the way back too, even though (or perhaps especially because) we had also stopped at the Krispy Kreme in Milford, bought nine dozen donuts, and eaten a dozen of them. A small sign on the wall behind the register proclaims, on behalf of some Hartford TV station, that this is the best diner in Connecticut, and (if only because it's not New Jersey) I believe it. Mm, good. Open 24 hours.