Railways are cheap if you can pick the route freely and optimise for low cost only.
Every extra constraint adds cost. If you have to cross a mountain range or a river at a specific point, you pay. If you have to enter a city, you pay. If you have to enter a city along a specific route, you pay. If you have to subsidise another project to get someone along the path to accept the proposal, you pay. If you have to support a certain speed, you pay. And so on.
In this case the only constraints appear to be that it has to cross the Bering straits where the tunnel is cheap (well, "cheap") and that it has to connect to the railway networks in each country it passes through. That's a lot of liberty for >10⁷ meters.
It's two days nonstop for a high-speed train or four if the route is optimised for cost of construction.
