This is mostly true, but in fact the route currently used by the California Zephyr through the Rockies (Salt Lake City - Soldier Summit - Moffat Tunnel - Denver) was built by the Denver and Rio Grande Western Railroad or its predecessors [1]; the section in question was completed no earlier than 1883.

The more famous railroad finished in 1869 takes a more northerly route, through Ogden instead of Provo, and through Rawlins, Laramie, and Cheyenne in Wyoming instead of adjacent to I-70 through Grand Junction and Denver.

The mountainous sections of the Zephyr, specifically the crossings of the Rockies and the Sierras, are indeed the least competitive vs. driving. Based off of the official timetable [2], the Zephyr takes:

~9 hours between Chicago and Omaha vs. 7.5 hours driving

~8.5 hours between Omaha and Denver vs. 7.5 hours driving

~15 hours between Denver and Salt Lake City vs. 8 hours driving

9 hours between Salt Lake City and Reno vs. 7.5 hours driving

7.5 hours between Reno and Emeryville vs. hours 3.5 driving

Lengthening the driving time to include breaks, the train is viable between Chicago and Denver, and between Salt Lake and Reno, but is nearly twice as slow as driving on other parts of the route.

[1] https://en.wikipedia.org/wiki/Denver_and_Rio_Grande_Western_... [2] https://www.amtrak.com/content/dam/projects/dotcom/english/p...

Stating the obvious here, I know, but Chicago-Denver has so many flights it's not even funny, United, American, Spirit, Southwest, Frontier, well over twenty flights any day.

The nominal max speed of Amtrak trains is about 79mph [1]. When I was traveling up I-57 in IL (speed limit 70mph), the Amtrak train was going almost identically in speed.

Most of the class I railroad track was built to handle high speed (by early 20th century standards) traffic, so trains can generally reach 70-90mph (although not so much in suburban or mountainous areas) with little modification. The difficulty is that there's no economic reason to push freight trains to high speed, and mixing higher-speed passenger and low-speed freight creates scheduling conflicts, so passenger speed tends not to hit its top speeds.

[1] On freight track, that is. The NEC, which is all passenger traffic and all-electric, has higher nominal speed: even the "slow" NE regionals will hit 100mph on the empty track in MD.

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...

There aren't no sections, but there are certainly very few that beat 70mph...for example, IIRC the Amtrak Downeaster hits 70-80 at at least one point (the speed limit of the train is 79mph throughout the route)...for like a mile or two as it crosses the Scarborough marsh ;)