[sabj]

One reason for these tough requirements is that a very large portion of U.S. passenger rail routes go over track owned (and used) by freight rail companies. Therefore, passenger train collisions and other such risks are weighed not only against other passenger trains, but also heavier freight trains.

This is one reason why U.S. trains have (relatively) 'harsher' requirements than other countries where track is not shared to such a high extent.

Separately, this degree of track-sharing is one of the reasons why average passenger rail journey speed has decreased in many places over a 100-year time period!

Obviously, in both cases, there are many many factors involved - but, an important element.