Of course, we can argue that there are network effects or natural monopoly effects for fixed infrastructure like roads and rails, and thus there must be a public role. However policy rarely seems to remain at this reasonable position and instead quickly expands into something altogether different.
Aren't all modes of transportation in the US either subsidized or public-owned to some degree? We haven't arbitrarily picked one; we picked them all.
Air travel is maybe the least subsidized, though? Essential Air Service is probably the main thing? Long-distance bus like Greyhound is only minimally subsidized too.
But local transit (bus & rail), and regional and long-distance rail are all subsidized or publicly owned in the US. Most roads are publicly-owned, either locally or by the federal government. Long-distance bus and rail are actually unusual in how little they're subsidized.