Question: why does the rail have to follow the highway grade? It could go at a more gentle grade easily enough.
It's a huge amount of concrete pour for sinking the gentler rail grades into retaining-walled cuts and elevations. And, when the grade difference is significant...outright MOAR TUNNEL
The NEC FUTURE Commission did a lot of MOAR TUNNEL
on its various Shoreline bypass crayon alignments along an Inland route of I-84 and the Mass Pike medians, because the highway grades were so much stiffer than the allowable rail grades. So it does lead to lots of cost blowouts in the real world.
EDIT: Don't forget as well, curve allowances hurt when you're trying to shiv an 80-160 MPH design rail line along a 65-75 MPH design highway curve. So you end up MOAR TUNNELING
a lot around banking curves as well.