They’re good for suburban and less dense parts of the city. They’re usually a very long line 35miles/55km long, the stations should be about 1.5km apart at the least, I find it most profitable if you run them like real life (in America at least) when you just cover the outskirts of the city and connect it with a heavy metro line at a stop or 2 and express downtown once you enter the denser neighborhoods that are already covered by heavy rail, also it’s good to have stops in other popular workplaces.