[andyv133]

To add to this, the FRA regulates track speed based upon condition of the track and some other requirements. Track condition is typically quite good (rail companies really want to avoid derailments), but one of the biggest impediments is grade crossings. The faster the track, the more extensive protection needed to keep trains and cars separate. Table 42 [1] here gives the requirements. Most of the mainline track is Class 4. You'll notice to hit 125 mph every grade crossing needs a full barrier between the car and train. And faster that that, complete grade separation (ie, bridge). US rail was not designed with this in mind and while they close a grade crossing any chance they get, they're still a major hindrance to high speed operation.

[1] https://safety.fhwa.dot.gov/hsip/xings/com_roaduser/07010/se...