I try to keep passenger train transit times down to 5 minutes or less, above 5 I can’t get enough peeps to make a profit, just add another train on the same route to lower the time.
Have also noticed some routes take a few years before the peeps get in the habit of using the line, one advantage of starting out in the 1800’s it’s easy to make enough extra money to wait out the loss until a new line is profitable.