[saalweachter]

It also becomes tricky because adding more trains (or more train cars) at different times does different things.

A train every hour is convenient for some purposes, but for others you just want a bunch of trains early in the morning and later in the evening.

[bombcar]

For commuter rail inside a city it's nice to know "the trains run every X minutes so I can always get one" - between cities it can be more clumped around commuter times.

You also run into trainset issues where you want to run 5 trains in one direction and 5 back in the evening, which will require 5 trainsets, but if you run them back and forth you could do more trips with less trainsets, but some would be running off-peak (and in the worst case, nearly empty, but getting into position).