The entire NYC subway, the busiest system in the country, has 660 miles of track. NYC has a GDP of $1.3tn and the subway system has a big hand in making that possible. So that's actually a pretty good RoI. It'll go a lot further (haha!) in a less expensive city.
But that also goes back to the original comment about spending the surplus wisely.
Given the current costs for NYC to expand its subway, about 500 miles of track given the current costs per mile.