We would like to consider the possibility of quaint rail accessible
towns between the two.
To me, towns don't get much more quaint than Gengenbach (population 11,000). Tübingen is nice but larger (population around 90,000). The Black Forest railway from Gengenbach headed in the opposite direction of Strasbourg will bring you to more quaint towns. Are you planning for a quick stop along the way or planning to spend a couple of nights at a base and explore? For either, I think Gengenbach fits the bill.
Stuttgart is a little more out of the way, but it is bookended by gorgeous Esslingen (whose town center was not bombed in WWII because its industrial area was out by the river instead of close to town) and Ludwigsburg (with its Baroque Residential Palace).