In a sense, ultralight trains - ultralight compared to American standards - already exist and are in wide use outside the USA…
https://bikeeastbay.org/rail/fra.html
http://zierke.com/shasta_route/sidenotes/safetyconcept-2005-10-23.html
http://zierke.com/shasta_route/pages/15regulation.html
[Note that the author of the latter two pages uses “amazing” sarcastically.]