Also, your position seems to be that the costs of building about 20,000 miles of track would be hundreds of billions of dollars.
Only hundreds of billions is quite a lowball estimate. $5M per mile gets you to $100B. In an uninhabited area where you already have the ROW, you can perhaps lay a second track next to existing track for that price. If you have to acquire new land in an urban area it is a whooole lot more than that.
So the value of Class Is’ track is many times higher than their market capitalization?
That makes no sense; if the value of a company’s hard assets is many times the company’s market value, something’s really wrong.
It makes a great deal of sense (assuming that when you say "value" you mean "cost to replace/replicate", as opposed to "price someone would be willing to pay to buy the existing item.")
It means the Class Is own a legacy asset which is cost-effective to maintain, but would not be cost-effective to build from scratch today.
When is the last time someone laid, say, more than 100 miles of new track?
In North America, the three most recent examples I can think of are the Powder River Basin -- single track laid in new territory in the 1970s, double track added in the 1990s; BC Rail's projects to build to Fort St. James and Fort Nelson in the 60s (completed, and still operating though not minting a ton of money); and BC rail's attempt to build to Dease Lake and on toward Alaska in the early 70s (suspended unfinished 1977.) One of those three was a good business investment. A second was a good political/societal move, and at least covers it operating costs, after being built at government expense. The third was an expensive flop, and would have been much more expensive, though less of a flop, had it been completed.
When is the last time someone laid 1000 miles of track?
In North America, the most recent project of that size I can think of is CN's line to Churchill, 90 years ago. In the US, is there an example more recent than the Milwaukee Road and the Western Pacific, 110 years ago?
It has been done in China recently, and in Japan, France, and Russia somewhat less recently - at large government expense; I have seen no figures whether the lines have made back their construction costs.
Similarly, re Amazon -- the company's market value is tied to how much merchandise they can move in a year, not to the resale value of their warehouses and trucks, which will depreciate.