A good early game tactic with a passenger service between a small number of towns is to deliberately starve them of one of the service types in order to increase traffic between those towns.
For example, connecting two towns, and only delivering industrial goods to one of them and only delivering commercial goods to the other one, will create an incentive for passengers to go working and shopping to a different town they live at, making passenger trains between those two towns very profitable. As the two towns are connected, the zones are developing (otherwise they would stall, for example industry cannot increase without enough workers)
However, is this a good long-term strategy? Did someone crunch the numbers or made advanced benchmarks, or at least has a good understanding of the mechanics? Late game, when most of the towns and industries are connected, would it make sense to starve a few towns of some types of goods, in order to increase passenger traffic, or would the growth of the town be slowed down enough to not make it worth it?