Nobody does infrastructure like the Chinese and at 1/3 US/EU cost.
Cost is often lower, when you are using forced labor.
Both India and China are building “infrastructure” at a frenetic and highly subsidized pace in the Himalaya primarily to be better able to block each others invasion routes and make stronger claims on hugely important Himalayan watersheds.
Tunnel work is pretty technical, especially when it’s done at alpine altitudes. I’m pretty sure on a project like this the labor is well-trained, and well-paid for China. They’re not busing in the Uyghurs.
China will saddle Nepal with an immense and unrepayable debt while promising a world of free trade benefits. Then China will foreclose assets and use the railroad to continue their long range plan of subsuming Nepal into China, just as they are doing in Tibet. Look at the recently completed international airport in Pokhara, Nepal, financed and shoddily built by China. This will be more of the same.