Pretty much no one. But Chicago to St. Louis? Or Chicago to Minneapolis? Or Charlotte to Atlanta? Or Dallas to Houston? Or LA to San Francisco? Etc/etc? Lots of people. That's here high-speed rail investment helps. High-speed planes are fine, until it snows in Chicago, or thunderstorms in Atlanta, or high winds in Denver and then the whole national air system gets screwed up. Alternatives are good.
But if you add up all the people who want to travel between Chicago and St Louis in a year, and then multiply by the hour-ish that they might be able to save by catching a high speed train instead of driving, and then divide by the tens of billions that such a railway line would cost to build and maintain, then... is it really a sensible use of funds per man-hour saved?
Also, you make a good high speed railway, and the demand will appear. In my coutry also nobody used the train, untill they were modernized, suddenly the word started to spread that it was nice, and now too many people ride the trains and we need more.
How many hours are spent stuck in traffic? How much loss is that to society.
Perhaps people would start moving closer along the rail lines, now that frees up land from sprawl. Etc. The same rail line could carry goods (fewer trucks on the road). The rail line could be electrified, now pollution will be down and so on.
Billions of dollars is not all that expensive. The US is a rich country. First time realized this when I saw a small town with 30K population easily built a new high school for 150 million USD - compared to that, the "billions" does not sound all that big if it reconfigures the economy at that extent.