In addition to the counterintuitive traffic congestion stuff, you also have to consider cost. Bridges ain't cheap. Metro isn't either... but you could make a lot of improvements to Metro for the price of a bridge.
The 7000 series has fortunately been a godsend for Metro. Now it just needs a miracle for each of: power supply, rail insulators, water leaks, automated control, and the union contract/pension.
The 7000 series has fortunately been a godsend for Metro. Now it just needs a miracle for each of: power supply, rail insulators, water leaks, automated control, and the union contract/pension.