I don't know how technically viable hyperloop is, but it seems especially well suited for the United States.

Investing in a hyperloop system doesn't make as much sense in Europe or Japan for a number of reasons:

1. European/Japanese cities are closer together, so Hyperloop's long acceleration times are a larger relative penalty in terms of speed. The existing HSR systems reach their lower top speeds more quickly.

2. Most European countries and Japan already have decent HSR systems and are set to decline in population. Big new infrastructure projects tend not to make as much sense when populations are declining and the infrastructure cost : population ratio is increasing by default.

3. Existing HSR systems create a natural political enemy for Hyperloop proposals. For most countries, having HSR and Hyperloop doesn't make sense.

In contrast, the US seems far better suited:

1. The US is set for a massive population increase, requiring large new investments in transportation infrastructure in any case.

2. The US has lots of large but far-flung cities, so long acceleration times are not as much of a relative penalty.

3. The US has little existing HSR to act as a competitor. The political class has expressed interest in increasing passenger rail infrastructure.

4. Hyperloop is proposed to carry automobiles. Low walkability of US towns is the big killer of intercity passenger rail in the US. Taking HSR might be faster than driving, but in addition to other benefits, driving saves money on having to rent a car when you reach the destination city.

Another possible early adopter is China (because they still need more transport infrastructure, land acquisition is a trivial problem for the Communist party, and they have a larger area, mitigating the slow acceleration problem.) I see China as less likely than the US because they do have a fairly large HSR system and it is expanding quickly. Also, China is set for population decline within a few decades, although they have some decades of slow growth left.

Russia is another possible candidate. Admittedly they have the declining population problem, but they still need more transport infrastructure and they have several big, far-flung cities. The current Russian transportation system is quite unsafe, so they could be expected to be willing to invest in big new projects. The slow acceleration problem would again be mitigated by Russia's large size.