Past a certain point the cost of new marginal capacity is hundreds of millions, or billions of dollars, or even tens of billions of dollars in new metro, bus rapid transit, etc. That’s money that could be invested in welfare, housing, healthcare, etc. And it has to come from somewhere. And this is before you consider that it takes a while to build such projects.

Guangzhou had to pull the plug on free transit during the 2010 Asian Games because queue times for the metro increased by half an hour just to get into the station, let alone board a train. What’s the use of a free train if you can’t board it?