Introduction

Welcome to the blog. I've looked over maps of many of the largest cities in the U.S. which have little or no rapid transit, and carved out some possibilities for them. Here are the rules/notes:

1. Existing train tracks. All the new routes only use currently developed rail lines. Many of these may be active (or occasional) freight routes, which adds expense by the added complications to comply with the FRA shared track requirements. However, this is more than made up by using existing right-of-ways and minimizing tunnels, elevated lines, and land acquisition. 
2. Mile spacing. Stations are at least one mile from their neighbors on the same line, except in the city center, where they can be as close as half a mile.
3. Through-running trains. Whenever possible, the lines pass through the city center rather than terminating in the core. This allows for reverse- and through-commuting and minimizing storage yard requirements in locations where there is the least room.
4. Station colors. White with black circle stations service more than one line. Extra large stations are either city centers or major transfer points. Solid colored circles are served by a single line during all operating hours. Small white circles with colored backgrounds are for outlying communities and are served only during peak hours. Tick marks are existing (in real life) light rail stations.
5. Line colors. Mostly anything goes, with a fair amount of whim and chance. But there are 2 rules: black lines are used where the right-of-way is intact (or nearly so), but the tracks are missing or impassible, and so would require a bit more to get up to working order; grey lines are used for an existing (in real life) commuter rail system.