there's an abandoned line running through the vista...you can still see the infrastructure (a few overpasses over the line, a few tunnels.) maybe that could be reused somehow?
to the OP: interesting ideas. i've had a few ideas myself, not as compelte a system as yours though, and sort of based on the light rail in Manchester, England (start out simple and expand.) subways are expensive, so i don't see that happening...it'd be nice though.
Rosewood/N. Beltline (switches with Assembly and Forest Drive/Taylor;) possible extension to stadium/fairgrounds and Olympia
Assembly - part of it makes a western loop around West Columbia/Cayce; the other part goes north, splitting at N. Main and Broad River (which goes to Harbison)
now that there's a loop, make the inner lines:
Forest Drive/Taylor - connects with N. Beltline, Harden and Assembly
a Harden line for Five Points access, and then go from there on extending it out.
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