I'd guess that's a classic case of economies of scale, although inflated property costs and government corruption could also be playing a role. However, compare to China(wiki):
> "The HSR network reached just under 38,000 km (24,000 mi) in total length by the end of 2020. The HSR building boom continues with the HSR network set to reach 70,000 km (43,000 mi) in 2035."
At that scale, costs per km would be expected to drop off on a steep curve, due to economies of scale, distribution of initial investment costs over more km of rail, etc.
Having your home demolished for a development project in China used to be like hitting the jackpot, because it meant your cheap old property will often be exchanged for expensive new apartment(s) in addition to massive cash compensation. There’s literally a nascent social class in China called 拆迁户 (relocated households, roughly) known for getting ridiculously rich overnight. A shitty old family home in the right place in Shanghai could net you something like $10-20m, for instance.
I heard that compensation has been vastly scaled back since a year or two ago, but I don’t know the details.
> "The HSR network reached just under 38,000 km (24,000 mi) in total length by the end of 2020. The HSR building boom continues with the HSR network set to reach 70,000 km (43,000 mi) in 2035."
At that scale, costs per km would be expected to drop off on a steep curve, due to economies of scale, distribution of initial investment costs over more km of rail, etc.