The US railroad system at its peak in 1917 according to Wikipedia had a total length of 428.180 km, today 220.044 km. So a lot of it was probably scrapped, many stuff also in favor for cars. I mean car lobbying did its thing in many cities to prevent the installation of public transport for sure, which is why many cities are they way today they are.
But for me the epitome of car delusions is not lack of railroads or other stuff, but the Las Vegas Loop. That's just bonkers, totally idiotic, something not even Mad Magazine would have come up with. That's what civil engineers really need to rip out of their minds.
It's never too late to start laying out new stuff. And yup, country side you will never have lanes to everywhere with public transport, totally fine. But for big cities it's quite an improvement in many aspects if it is there and works.
Let's take for example Norway. Comparable in size to California, but only 5 million people. So of course cars are there a necessity as well.
Let's say Cindy lives in Wichita. (I'm not sure where she is in Kansas, so this is no reflection on her taste or judgment...) She decides she'd enjoy a trip to Chicago. If you could travel in a perfectly straight line from Wichita to Chicago that's a trip of nearly 1,000 miles. Even high-speed trains don't
average those high speeds, and the chances of non-stop service from Wichita to Chicago are slim. So, let's say we have a really fast train and super-efficient boarding and deboarding and luggage/freight loading and unloading (and maybe some fueling that is done by a NASCAR pit crew) and we can average 80 mph over that (perfectly straight) 1,000 miles. That's still an unrealistically optimistic 12.5 hours of travel. Via air -- about 2.5 hours. Of course, the route cannot be perfectly straight, either, so that adds even more time. How many people are going to give up a long day to travel by train when, with the same amount of misery, they can fly?
Train travel is quite popular here in places where the numbers make sense. San Diego to LA is a popular route. The California Starlight route, from Seattle to LA, is well-traveled, but far more because of the views (awesome) than practical travel. Put it this way; I've never met anyone who took that trip twice. Come to think of it, I haven't met anyone who took the round-trip! The East Coast is well served, and cities are close enough together to make the Washington DC - NYC corridor a highly ridden and, indeed, profitable part of our national passenger rail company, Amtrak.
What we do superbly with rail in the US is move freight, and that is really what our system is built to support.