Sure, the US will have cheap workers, but high skilled migrants will prefer other countries. This is not just going to hit farm workers after all.

By detain you mean this right:

You pay 10% for the product when imported. However there are a lot of people, work and capital involved to transport the product and actually sell it to you.

Also a lot is also not imported, but made in the US. That is especially true for a lot of food, electricity, gasoline and even quite a bit of actual physical products.

This is generally true. HSR is only really needed to replace long car and plane trips, which are not that common. In daily life regional rail is much more usefull. That combined with local public transport, cycling and walking actually can replace a car. HSR alone can not. Obviously when you built a new line going for hsr is an actually good idea, as it does not cost that much more. But there is a lot of railway infrastructure around already. So adding trains on it for regional rail, electrifying it and so forth would really help. The US is probably the most obvious thanks to having a massive freight rail network, mostly without passenger trains, but a lot of countries have old unused tracks.

Sources: https://en.wikipedia.org/wiki/China_Railway_High-speed#Ridership

And that happens a lot, just look at Chiba:

Most of the lines are used fairly well. Overall ridership of the network was 3.2billion trips last year. It is still growing.

As for the economics, it is infrastructure, which is going to last for a century or more. It obviously requires upgrades, but having a fast reliable, green form of transport between a countries large cities has a lot of advantages. Not the least are indirect economic advantages. Like for example making business trips easier, but also tourism. That is why Japan, South Korea and Western European countries built hsr as well.

That also means taking on debt is somewhat sensible, as long as economic growth from the better connections is bigger then the cost of the debt. That is honestly just running the country like a business.

The netherlands are pumped dry anyway…