Speaking as someone who lives near a rail hub (and indeed, one of the busiest transshipment rail hubs in the world) freight rails absolutely do cross other transit arteries, and enter urban cores where the people are. Indeed, in the U.S., most passenger lines actually run on freight lines (including almost all of the Amtrack network excluding a few lines in the Northeast).

100mph is not a "high-speed" train. It's just a train. Freight engines are capable of going up to 80pm with cars, and over 100mph without (but don't for safety and operational reasons). Freight lines are designed for cost efficiency, not speeds, but generally unless your 100mph train is using specialized wheels, it will run just fine on a freight line, though it will need to slow down for turns due to the shortened turn radius on most freight lines.