The rules for my lines inside a city.
1. |The line should not overlap with another line. If a transfer is required, build two stops as close together as possible or use a bus/tram station. This avoids congestion.
2. Keep frequency under 60 seconds. On some routes, this requires 20 or more horse trams/stagecoaches. Keeping such a low arrival time will ensure that just about everyone will choose public transport over walking.
3. The line should include at least 1 intercity transfer stop. If it does not, then it needs to have 2 or more land use zones within the stops.
Most of my lines start at the center of the city and move to the outskirts. This means most lines have the commercial zone included along with at least 1 more zone. Most of the lines also end at a station that delivers traffic to another city.