Source: Census TIGER/Line Data
Tools: ArcGIS

Top 5 Counties:
Cook County, Illinois - 999.9 miles
San Bernardino County, California - 895.9 miles
St. Louis County, Minnesota - 631.0 miles
Franklin County, Ohio - 604.0 miles
Wayne County, Michigan - 575.5 miles

That’s an interesting way to present the data.

First thing popped into my head is how railways have slope limit both uphill and downhill. To traverse a slope will require way more rails to loop and/or zigzags. So rails in mountainous region will have significantly more miles to cover the same distance between two points than in a flat region. I wonder if there’s enough data to change the measure from plain miles to actual distance covered. That would make an interesting comparison.

Doesn't this favor larger regions?

So there's more train rails(meaning a higher chance of derailment) in more populated regions? Sounds like a recipe for disaster

Yes. There's several counties in the southwest that have next to nothing in them but because a railroad crosses them they are counted as dark green.

Needs to be normalized as a ratio of miles to relative county size.

Yes that is how trains work (they bring stuff from place to place, aka where the people live)

Former Southside Chicago resident here. Can confirm the absurd amount of tracks we have in Chicagoland