It depends on the commuter rail. The S-Bahn in Germany is sort of 'commuter rail' but also gets regular use for other types of trips. And actual commuter-specific train lines tend to go on just general tracks that are used for other lines as well, so it's not a huge deal.
It's just that US-style development -- more or less mandated by the government -- makes public transit in general infeasible. For example, in Munich where I lived, there are suburbs nearby surrounded completely by farmland, where the whole town is still a handful of minutes by bike to the train station. That means that even in these sleepy little towns, it's dense enough to where a car is unnecessary. In the US, that kind of development is a rarity, because the local zoning makes it literally illegal, and the roads have been designed to be hostile to anything that's not a car.
That kind of town is vanishingly rare in the US. The only ones I can think of in California where a meaningful amount of the town is within walking distance of a train station are Davis and Chico, both university towns. Chico only has one train per day and it comes at 3 in the morning :-/
That's not true at all, those towns are common across the Midwest and the South of the US. The only difference is that the rails are freight, so you have to use the highway to get anywhere.
I grew up in several of those towns. I don't think I lived more than a 15 minute bike ride from the railroad tracks until I was in my 20s. If Wikipedia can be trusted, upon construction over 80% of the farms in the Midwest Corn Belt were within 5 miles of a railroad (which is an area roughly 5 times the size of Germany). I never lived close to a passenger train station until I moved to a big city.
There are lots of small towns where there's a semi-dense urban core with a grocery store or two, a couple bars, a few restaurants, apartments and some shops. A lot of them even exist within a fairly small radius of major cities. Nobody builds rail anyways. My personal guess is that the major city doesn't want to pay for the rail, and the sleepy towns can't afford to pay for the rail. That's why most of what we see is major cities connecting, because they get enough economic activity out of those rails to justify the connection.
This sort of density pattern is present in Colombia. The government taxes unbuilt land on a lot at a higher rate than built land, so this incentivizes having houses share a wall, which is the norm.
It's just that US-style development -- more or less mandated by the government -- makes public transit in general infeasible. For example, in Munich where I lived, there are suburbs nearby surrounded completely by farmland, where the whole town is still a handful of minutes by bike to the train station. That means that even in these sleepy little towns, it's dense enough to where a car is unnecessary. In the US, that kind of development is a rarity, because the local zoning makes it literally illegal, and the roads have been designed to be hostile to anything that's not a car.