The hotter the ambient temperature is the slower trains run because of the risks of sun kinks derailing a train. When you know the rolling stock is going to be exclusively high-performance electric-multiple-units, you can design the track more aggressively than you can for general-purpose track, which has to account for the worst case scenario and can’t be optimised for one type of rolling stock. It's not just power-to-weight, but you have a lot more traction-to-weight. This is completely different from freight, where you have maybe 100 wagons which are just dead-weight being hauled by a locomotive, and it's only the loco's 8-12 wheels which are providing traction. (Last, but probably most importantly) HSR trains are almost exclusively electric-multiples, which means every axle is driven and providing traction. 7-12kw/tonne for conventional passenger trains) HS Train sets tend to have a higher power-weight ratio than other trains (18-25kw/tonne vs. The station sections/approaches need to be flat just like lower powered trains. This assumes you’re not having to stop at a station and accelerate away again, but most HSR is for inter-city express trains so doesn’t make many stops. If you’re running at 200mph you can deal with more slope than usual because of the momentum you carry into it. They don't stop very often and mostly cruise at top speed. High Speed Rail can tolerate higher gradients because:

Minecraft rails uphill