My goodness this is why you're clueless. The shorter the payback period, the lower the interest rate. Since you have admitted that VN is super POOR, then we can assume that the loan payback time is 30 years or more.
Now take a look at the current market rate for risk fee sovereign bond of Europe and see for yourself.
https://www.investing.com/rates-bonds/european-government-bonds?maturity_from=10&maturity_to=290
European countries are offering their bond at a little above or below 2% while America is offering 3.2%
Why would Japan let Vietnam borrow $60 billion at 0.5% when Japan can lend that to Europe at a risk free return of 2% or the U.S at 3.2% instead? This is why people of this forum take you as a clown when you're talking about Japan will lend a high risk low credit country like Vietnam a near zero interest humongous loan.
If VN wants to borrow $60 billion to pursue the retarded HSR project, then the only way VN can secure the fund is to issue bond to creditors. Vietnam's 10-year bond is already at 5.8% interest rate, now image what a 30-year bond rate would be,
20% may I say? LOL
This is why no matter how millionth of time VN wants to present this HSR crap, it won't ever materialize due to lack of fund.