There is a frequent assumption that engineering a line for higher speeds is much more expensive. It’s more nuanced than that. High speeds enable the use of momentum to climb hills because as v grows, 1/2 mv^2 >> mgh. This allows straighter alignments that hug terrain with steep (4+ percent) grades, with shorter and fewer bridges and tunnels than would be required for slower diesel technology. The resulting cost comparisons can be surprising and counter-intuitive.

Here is a profile comparison of Paris Lyon via legacy steam-era alignment (top) versus modern HSR alignment (bottom)
[www.railfaneurope.net]

Of course all the HSR authority’s Pacheco engineering to date involves extra long and expensive base tunnels with gentle grades, precisely the opposite of what they ought to have done.