Trains can't pass each other, so you will need multiple lines with lots of trains, and they don't serve all transport needs unless you manage to get rail outside all shops for instance.
Roads outside of shops is a small minority of all roads, and we are currently undergoing a change in how city planning is handling commercial districts, with movements towards the edges or online.
There are however major problems with train networks, one being that lack of redundancy. With road maintenance you can shutdown a major highway and redirect traffic temporary to smaller roads with significant reduction in speed. With trains you either have the option to go around the maintenance, or you replace the train temporarily with road based travel.
A second big problem with train infrastructure is the lack of an obvious upgrade path. With roads you build a new highway when security, air pollution and noise become too severe for smaller towns around the old road, and the better road also adds higher capacity and faster allowed speeds for transportation. There are so many incentives to upgrade old roads, and with an increasing population it makes traffic heavier for old roads, speeding up demand for better and bigger roads that runs outside the towns.
For rails, there is mostly just an economic argument in favor of building more. The reduction in car travel is not significant enough compared to building a new road outside the limits of the smaller towns.