the train length restriction is actually a lot more compilcated
amtrak doesn’t actually own most of its track, that would be private freight railroads. (since freight railroads own and maintain the track, they get priority which is why long distance trains are delayed)
in this case, the rightful owner of the track around denver is the BNSF railroad
what makes denver complicated is that it’s a terminal station instead of a thru station, which means the track ends at the station and the train needs to back up (kinda like parking a car in front of a wall at a wawa or 7-11 or whatever)
since we are attaching private cars to the end of the train, and the private cars are unsuitable to be at the front of the train when it reaches the main line (aka the where it’s shared w/ freight), that complicates how trains with private cars can back out of denver.
first of all, this is how the westbound zephyr enters denver
this is how the westbound zephyr exits denver’s terminal station, should it want the seats to face forward the rest of the way there. it reverses following the green arrow, then uses a critical turnaround track in red (which may likely be the source of the restriction), then follows the yellow arrow to california
(there are other routes that the zephyr can use to reverse the train past denver, but this is the main one last time i checked a few years ago)
notably, the wye (aka turnaround) is owned by BSNF and more importantly, it goes thru their freight rail yard. in the world of passenger safety, freight trains operating in close proximity is usually a big nono
here’s a closeup, down the the exact track amtrak likely uses to turn around the zephyr. the blue circles show what is likely the 2 critical switches in which amtrak has to fit the train in between, the blue line shows the space physically available for amtrak to turn around. to the south circle is the switch to/from union station, and to the north circle is the switch to BSNF freight yard limits. (remember mixing passenger trains and freight trains is largely a nono).
(convienently, a freight train is are shown to the track to the left, but a standard north american freight car is about 140 feet long. a standard passenger railroad car is about 75-100 feet long.)
freight cars in the satellite image gives us an approximately 1400 feet to turn around the train, let’s say approx 1600 to be generous. (the actual train needs to be shorter than this!)
so here’s my math around the train length restrictions: the zephyr uses 16 superliner cars, 1-2 locomotives, plus we’re adding on 3 private rail cars here. 85*16=1360 for the superliners. the GE genesis locomotive is 69 feet (nice), so 1360+69=1429. the superdome is 80 feet, so let’s assume 80*3=160 and 160+1429=1589. which means the train length is around 1589 feet long, and i estimate the turnaround track length to be 1400-1600 feet. so there’s physically no way you can have a longer train unless if you can get the FRA (federal railroad administration) to change safety regulations to allow passenger trains in revenue service to enter active freight yards.
alternatively, there could also be this simpler answer: the longest passenger platform at denver union station is 1200 feet, the longest “emergency access” platform (low level) extends an extra 150 feet, which means the passenger accessible portion of trains are limited to 1350 feet. that said, you should be able to walk between train cars so platform length should be a non-problem. however, the maximum physical length of a train that union station can accommodate (from bumper to the nearest switch) is 1200-1600 feet depending on the platform, even though there won’t be a platform long enough for the back of the train.
of course, there’s nothing stopping you from decoupling (splitting) the train in half so the train can fit into more of the platform at union station. but that likely can’t be done for the turnaround because of the sheer amount of people/crew/approval needed to perform that manuver. :)
what complicates thing is that there isn’t an incentive to improve the turnaround situation, to the east is BNSF, to the west is Union Pacific. since these are competing private freight railroads, there isn’t a good incentive for them to connect with each other, which again is bad for amtrak.
amtrak does have permission (likely a waiver) to use the eastern most track in that specific yard, but it only when directly entering or leaving union station, not for turnaround. that track would not be useful for a turnaround either.
also the california zephyr is delayed by an average of 191 minutes by the time it reaches denver, so plan accordingly!
the preferred long distance route of private railcar owners is the empire builder to the north, and the southwest chief to the south, both of which originate from chicago and does NOT have these train length restructions! for example, here’s an empire builder train with 8 private cars: https://youtu.be/QOc17u75BKM (aka more people could had attended had a different route been chosen)
amtrak also updated their private car policies in 2018 which made things worse for private cars, and eliminated one-time trip charters so they are no longer a thing: https://www.bloomberg.com/news/articles/2019-03-25/can-you-buy-your-own-train-here-s-what-it-takes
i’m finding my info might (possibly) be outdated, the zephyr in 2021 now usually (but not always) reverses before entering denver going westbound. there is a different switching area within the UP yard to the west that amtrak sometimes use, complete with a road if it needs to decouple trains but get this, it’s coincidentally also about approx 1500-1700 feet. depending on which freight railroad has the availability the zephyr could use either one (it could reverse either before or leaving denver)
(i’ve also identified another switch point on the BNSF yard about 350 feet to the south, making this 1750 feet instead of my original 1400 feet measurement, which gives room for amtrak to attach a second locomotive needed to make this move, plus a more reasonable buffer)