What they did before cars were ubiquitous was "whistle stops" the train would whistle outside of town to indicate it was coming, and would stop for anybody at the platform otherwise it would continue on without stopping, much like how most buses operate today.

Smaller interurban lines also operated "flag stops" where it would stop when flagged down by a passenger otherwise it would keep going

With EMUs (electric multiple units) and modern signalling service could be brought to small towns fairly cheaply. Most small towns in the US are about 20 miles apart, so hourly service at ~50MPH could easily be provided with a single track mainline and passing sidings only at stations by having one train in each direction every 30 minutes.

The problem is so much money is invested in road infrastructure that investing a similar amount into an equally extensive rail network is simply unfathomable