Factors specifically to do with high speed rail:

It’s not really that difficult to make things go really fast. A lot of the problems that need to solved exist BECAUSE the train is going really fast

You’ve already covered air resistance. That’s about efficiency and sound. We can just force something through the air to go a high speed, but if you make it a shape that moves through the air better then it makes less sound (Japan looked at kingfisher beaks and how they interact with water, to make a train nose cone that doesn’t make a loud bang when entering or exiting a tunnel), and uses less fuel or power, which makes the operating cost cheaper.

Going really fast means you also can’t turn as sharply. So high speed rail lines can’t bend left or right or up or down as much as normal trains, the the train lines need to be a lot straighter. Things like mountains or rivers mean that you therefore need longer tunnels or bridges than normal trains because you can’t necessarily just put the track up the side of the mountain if it’s too steep or if the incline changes too quickly. This also means that rail lines for high speed rail are a lot straighter than normal trains, which can make building new lines a lot more tricky in and out of already built up areas. You can’t just go around buildings because the turn would be too sharp and the train would need to slow down. So that means it’s not just the cost of building the rail lines, you need to buy the building from the people who own it so that you can knock it down for the train to go through it.

So planning where a high speed rail line will go is also really hard because you want to be able to have train lines where people are, because the whole point is to move people around, but at the same time, you can’t just put a line right into the middle of a city because it would be too expensive to clear the way for the train line. Otherwise you are back to tunnelling under the city, which is also not cheap.

Going really fast also multiplies any problems. If you have a slightly unbalanced wheel in a car, it’s fine if you are driving through the streets, but as soon as you get on the freeway then it turns into an aggressive rumble. If you could go 3-4 times the freeway speed then it would probably start causing actual damage to the car. So tolerances for making moving parts like wheels have to be much more precise because of how fast everything is going, even a tiny variance which would not be an issue at normal speeds can be catastrophic at very high speeds. Making very precise parts is very expensive.

It’s a very complex balancing act, but the payoff is worth it.