From the publisher's website:
The Freedom City Atlas is a series of products intended to flesh out and further detail the award-winning Freedom City setting for Mutants & Masterminds.
In this series, we "zoom in" to look at different facets of Freedom City in detail, providing you with all the information you need to know about them to make them centerpieces in one or more adventures set in the city of heroes.
Freedom City Transit Authority details the subway system beneath Freedom City's streets. In addition to providing some local color and a backdrop for various encounters, you can work the FCTA directly into your M&M adventures. Now if you'll excuse me, I've got a train to catch!