why do motor vehicles and trains slow down when going uphill? Q and A

https://share.google/aimode/f044gsZsc0AEGLuK6

we have seen big trucks slow from 70 mph to 50 mph on hills on
interstetes - why dont they slow to 25 mph or 10 mph? must have
something to do with horsepower - whats the relationship[ between HP
and grade?