"Milk run" trains and Eurail

Is this pass accepted on what Rick Steves refers to as "milk run"
trains?

Absolutely!

First, "milk run" trains, called RB (Regionalbahn) in Germany, and, R, I think, in Austria, are the lowest (slowest) form of train transportation in these countries; they stop at every station. If they were not accepted, nothing would be.

Second, in Germany and Austria, there are no national rail trains (Bahn and ÖBB) that are not covered by a Eurail pass. Even the ICEs (Intercity Expresses) in Germany and RailJet in Austria accept the Eurail pass.

And you need to understand that the Eurail Pass provides flexibility and not necessarily economy. The pass could be more expensive.

What routes will you be taking on your trip?

Mostly all, there are a few private lines, like the well known Circumvesuvia in Naples that would not be covered, and a few rail lines in Switzerland, and others scattered around.

The other thing to watch is coverage in urban and suburban areas, most commuter trains are operated by the national operator, like the s-Bahn in Munich, are included, but the U-Bahn, operated by a local metro organization, is not. There are also quirks like the Leonardo Express being covered, but only if you hold a First Class pass.

If I recall, the material that comes with the pass does a good job of pointing out exceptions, but I have not found it useful to have a pass in some 15 years.