Looking for a Halfway point between Heidlberg and Strasbourg

Heidelberg to Strasbourg is only 2 hours. And there aren't really any places in between that are more interesting than Heidelberg and Strasbourg themselves. I would just spend a day more in either of them and just go straight between them.

In Germany, good options include either Baden-Baden or one of the small towns on the edge of the Black Forest, such as Gernsbach or (a little south of Strasbourg) Gengenbach. In France, consider one of the smaller towns in Alsace, such as Wissembourg or Haguenau.

Looking at the trains, I don’t see anywhere as interesting as Strasbourg as a stop on the way.

If it's just for a few hours to break up the journey, the only practical stops would be dumpy Bruchsal and Karslruhe, both of which have good palace visits. Train stops in both, though I don't know if either station has lockers.

It depends a lot on which train you plan to take. There are three reasonable routes, left or right side of the river, or up the middle of the valley. If you go up the right side of the river you have Karlsruhe. The train station there is an easy walk, through the old historic town center, to the Schloss and park. The schloss has been turned into a museum, which is exceptionally good with bronze and iron age displays, including a whole room of Etruscan stuff. The park is especially good in the spring.

On the left side of the river you have Speyer. If you have any interest in local history or cars, aircraft, boats, spacecraft, etc. this should be your destination. The cathedral is famous for having the largest number of Holy Roman Emperors buried there. The Technical Museum is, in my opinion, one of the best museums in all of Europe, and the small Celtic museum is very interesting. Plus it's a good town to walk through, the only issue being the train station is a way out from the old town center; but there are busses.

Up the valley you have all the local wine towns, to include places like Bad Durkheim and Neustadt, both of which draw tourists from all over Europe, and smaller little boutique places like Deidesheim or Freinsheim, 1200 year old walled cities. You also have Landau, which has a very good Roman Museum and a Vauban fort that's mostly intact and very walkable (now a garden).

If you're going to be in Karlsruhe , that is an interesting city, nearby is Rastatt, the ride from Karlsruhe to Rastatt.

That place I suggest if you want to see the Schloss and its esoteric historical topic. Very recommendable seeing that Schloss and its singular exhibit, I was there as a day trip once ages ago, ie 1989. Most likely it has expanded.

I'm not a city person. I once spent an afternoon in Baden-Baden and was frankly unimpressed, but maybe that just me - too big, too "industrial", too touristy. I'd prefer someplace smaller, "quainter". Gengenbach wouldn't be too bad, but a little to close to Strasbourg, almost a suburb.

Karlsruhe is more in between, but maybe also too much of a city for me ("NYC, a nice place to visit but I wouldn't want to live there"). However, just an hour outside Karlsruhe by "streetcar" is a little Black Forest spa¹
town called Bad Herrenalb. Check it out. I've spent weeks there since 2001. That's where I'd go.

1. But a word of warning, it's a traditional German spa; the sauna portion is "Kleiderfrei".

first time I have ever heard Baden-Baden described as too "industrial".

My memory - and I am in Baden-Baden every year to go to the round shopping centre - is of tree covered streets, nice houses, a bit of traffic on the B road, and a refined and upper class centre.

Never saw the industrial.

I've never seen Gengenbach described as a suburb of Strasbourg. Good grief - between the two are the Rhine, Kiel, Offenburg, and about 40 km of countryside. Have you ever been to Gengenbach, Lee?

Going from Heidelberg to Strasbourg by train you probably have to change train in Karlsruhe

Why not make a stopover in Karlsruhe?

No, I haven't actually stayed in Gengenbach, but I have gone past it four times on the train between Offenburg and Hausach. I've also spent some time in Offenburg, Lahr, and Reichenbach - my German relatives live in a village outside of Lahr - as well as a lot of time in and around Bad Herrenalb (including Karlsruhe and Baden-Baden). I kind of know the area.

And I have looked at maps. The common point for trips between Strasbourg and Heidelberg or Gengenbach is Appenweier. (That's from Heidelberg, if you are going by regional train; if by long distance train it's Offenburg, which is closer to Gengenbach.) It's about 2 hrs by train from Heidelberg to Appenweier and 26 minutes by train from Appenweier to Strasbourg. That's why I say Gengenbach is nowhere near the halfway point, Karlsruhe is closer to half way.

Friedrichsbad - including the brush massage - in Baden-Baden. I went pre-covid and am finally heading back this year!