In case you plan on updating these, I would love love love an overlay to add in state lines. For example, it appears that it would be technically possible to run a train from my hometown of Carbondale, IL to Saint Louis (blows my mind that it's not a thing) but putting in the overlay and being able to zoom and pretend make that route would be awesome.
This can be prototyped pretty quickly using Leaflet to serve the zoom-able tile map and topojson to reduce the size of the overlay data (in this case, train tracks and stations). I haven't tried topojson for a map of the lower 48, but I was getting a 80% reduction in data file size when using it on parcel data. This way, instead of rolling your own tile layer with the data which is a little bit of an involved process, you can just load a topojson file with the data and draw it as a vector layer.
I'm sure there are limitations that make this much more difficult than I'm expressing due to the volume of traffic that hits washingtonpost.com - I'm more remarking on this so if other people are looking to do data visualizations like this they know there is a good dynamic option out there they can try.
