Yes, the thing is just that the interstate system was in the end more expensive than similar rail infrastructure would have been (especially in maintenance).
And, well, yes, cars and trucks will still be necessary, but not nearly in the same amount.
I'd love to believe this quote, but light rail per mile is an order of magnitude more expensive than highway per mile. Do you have any citations I can look into?
$35million/mile for rail, $25million/mile for a single lane. Rail maintenance is cheaper, and ongoing costs are generally lower - you don't need police patrolling railroads.
Railroads are cool, and they have a lot of advantages, but we've got this huge investment in our existing system. And unfortunately, unlike self driving cars, there isn't a smooth transition. It's like switching from pc to mac, but all of your software is home made, and must be rewritten. You have a huge upfront investment to just get started, and you have to maintain the other system as well.
- much shorter stopping distances. This lets roads be "general access" in a way that rails have never been, even if you were to have a railway running past your house.
The issues are essentially based on (a) how many lanes your highway has, (b) how many complex interchanges you build (interchanges with many bridges tend to lead to a cost explosion), and (c) rail tracks are usually able to be used for decades without major maintenance, while highways usually need to get filled and fixed multiple times a year.
I only have a specific study for my city, where they were comparing the costs from fixing the streets in the cities from the busses running over them vs. building light rail instead, and in that study it turned out the light rail would be, if you also consider stuff like maintenance and accidents, cheaper.
And, well, yes, cars and trucks will still be necessary, but not nearly in the same amount.