Sure, but they do it with big machines that ride down the rails now instead of lining up thousands of men with sledge hammers.

I’d guess that overseas modern non-high speed trains could deal with it. The passengers might not put up with it though.

Well, back then the US had freshly banned slavery, so there was an ample workforce that could be hired for dirt cheap.

The Soviets and the Wehrmacht pulled off similar feats in WW2, but back then the rails and sleepers didn't have to be built to last many decades, so in addition to loads upon loads of forced labor from concentration camps and gulags, the work effort was massively reduced because easier technology could be used.