I'm not convinced this would be worthwhile in the St. Louis suburbs as they exist today. For one, low population densities naturally diminish the value proposition of rail. But, second, the existing auto infrastructure is so good that people would likely still choose to drive.
Yes, I agree that we should strive for higher population densities and that we shouldn't have invested so much in automobile infrastructure, but given that this is where we are, people will make rational decisions.
That's what I mean when I say that we're already too "spread out."
By the way, on the off chance anyone's interested, here's a map of those original streetcar lines:
https://www.google.com/maps/d/viewer?mid=1W809wsAmB9GR3ZvyII...
Melbourne has 4.6M people in 3800 sq mi.
It has a tram/light rail network of over 160 miles of track on 24 separate routes, and nearly two thousand tram stops. It runs a network of nearly 500 trams, and carries 200 million passengers a year.
https://en.wikipedia.org/wiki/Trams_in_Melbourne#/media/File...
To give scale to that map, the radius of those routes (you can see something of a circle around the downtown area) from the city center averages about 15 miles (East Malvern, Box Hill, etc), and stretches near 25 miles at points.
Using your St Louis example and recognizing that certainly, not all of the Melbourne urban area is serviced by light rail, well over 700 sq mi is 'very well served'.