These visualizations were generated for the Meyers Research Group at the University of Texas, Austin.
The image below shows the links between cities that have greater 300 passengers per day between them.
The data is from the Bureau of Transportation Statistics (via my wife). The image is generated using DIY maps to generate the nodes and the links and then grabbing that image from the screen. DIY maps takes an XML file which I generated from the data using a Python script.