At these speeds elevated sections require very large vertical curve radii, which are a tighter constraint than grade. The ramp won’t get very steep because if you let it, you’ll have climbed way too high.

In the hills and mountains, it’s easily 3.5 - 4%. No diesels need apply. The whole point of going steep is to avoid excessive $$$ civil works, although clearly that memo has not been well received at CHSRA.

In rolling terrain, momentum carries the train up steep grades in a way that regular speed trains can’t, because 1/2mv^2 >> mgh. 220 mph of kinetic energy equates to the potential energy needed to climb 1500 feet.