Let me educate you on what Wisconsin actually looks like since I grew up there.

50% of the population lives in Milwaukee, Green Bay, Madison, and Eau Claire. Those cities, respectively, are in the southeast, northeast, south-central, and north-central parts of the state. You could generously include Hudson and La Crosse in that list in the west and southwest parts of the state, respectively, if you wanted to add a couple of smaller-sized municipalities to tie the state together.

One more point of reference: Minneapolis/St. Paul in Minnesota lies ~40 miles west of Hudson and Rochester, Minnesota lies ~90 miles west of La Crosse.

I-94 connects Minneapolis/St. Paul, Hudson/Eau Claire then turns south through Madison and Milwaukee, then to Chicago. I-90 connects Rochester (Minnesota) through La Crosse then on through Madison and Milwaukee. Another interstate (I forget which) travels north/south and connects Green Bay, Milwaukee, and Chicago.

There is enough traffic along I-94 to justify a rail connection from Minneapolis/St. Paul/Hudson/Eau Claire/Madison/Milwaukee/Chicago. There is also probably enough traffic to justify a rail connection from Green Bay to Milwaukee as well.

Remember when I mentioned that 50% of the population lives in those cities? The other 50% of the population lives in rural areas and is generally involved in agriculture, forestry, or tourism, or support for those industries, and there are 2000+ small towns of 1000 population or less geographically distributed across the rest of Wisconsin, each typically 10-50 miles apart from each other. The towns did not arise like this by coincidence! This spacing actually creates an optimal geographic spanning tree across the state for the purpose of efficiently transporting food and forest products from rural areas to urban areas.

Now, something has to actually connect the farms/forests to the towns and the towns to bigger towns and the bigger towns to the urban centers in order to transport the food and forest products (and tourists). Traffic along the very rural links is extremely low; it is not uncommon to travel one of those links for 40-50 miles and not see anyone else on the link. So let's assume for a minute that it would be reasonable to build rail links, and let's assume (generously) that there are no hills in Wisconsin and that we can lay it out in a grid across the 250 miles x 250 miles to cover the southern 2/3rds of Wisconsin, with parallel rails separated by 10 miles:

* We need to build 12,500 miles of rail. Cost to build 1 mile of railroad track is $100,000, so the total cost is $1.25 billion dollars. Ok, not the end of the world.
* We need to build and operate the trains over these rails. Call it 100 trains. Data on the cost to operate a train is sparse, but call it $100/mile to operate a train. These trains would need to travel back and forth (conservatively) 4 times per day; 4 * 100 * 250 * 100 = $10,000,000 per day to operate, so the total cost per year just to operate the trains (disregarding maintenance) is another $3.65 billion dollars per year. _You can safely assume that the train operator - likely the state of Wisconsin - would lose $3 billion per year operating these trains_. Why? Remember when I mentioned just how lightly loaded the rural transportation links are? There would be some days those trains would operate with a grand total of 3 people on board. And oh, by the way, we still need roads covering the county in order to transport people and goods to the rail stations so we haven't eliminated the need for roads or vehicles, unless we're planning on increasing the rail grid by a factor of 10 each direction and now we've increased costs by 100x for both build-out and maintenance.

Also conservatively, we can estimate that the people in rural areas drive ~20,000,000 total miles per week across the whole rural population. (Farmers don't generally drive back and forth to town every day; maybe once or twice per week on a "busy" week.) Call it a $1/mile to operate a vehicle, that's $20,000,000 per week or about $1 billion per year to operate the vehicles that the rural people require anyway. Even if we built a 10-mile grid of trains the rural people would still need to drive that ~20,000,000 total miles per week to get back and forth to town, so this expense does not disappear.

Now, consider the fact that as soon as you travel more than 100-150 miles inland from the east or west coast the entire country generally falls into the situation that I described above, and repeat the exercise across ~2500 miles east/west and ~100 miles north/south to see how impractical it would be to blanked the country in rail.

Would it be feasible to replace highways with rail lines connecting SD/LA/SF/Sacramento? Absolutely. Amtrak should be operating rail along those routes and should be making a fortune doing so. Is it feasible to replace highways with rail lines across the US? There's just no way that can ever be economically feasible even if you factor in carbon externalities.