I think "fast enough" is determined by the market. Europe has a long tradition of fairly speedy main-line passenger trains, which HSR added to as a premium service, long ago fully grade-separated most main lines, and doesn't have a lot of long relatively slow freight trains to work around. It also has relatively short distances between cities and compact growth patterns. US is anything but on all counts outside of the NE Corridor. The only place (other than, very recently, part of Michigan) that Amtrak runs over 100 mph is the NEC - and practically all freight has been evicted from those lines. Where you have to mix it up with typical US freight on tracks owned and maintained by the freight railroads, 70-80 mph is about the maximum practical speed. Most of us drive that fast on the freeway (speed limits notwithstanding). So for us a great improvement would be to build enough bypasses and such to get cruising speeds of regional passenger trains up to 90 or so, with average speeds of 60-70, so they can be more competitive with driving.

It makes little sense outside of the NEC in the US to shoot for 200 kph (let alone 300+) as long as gasoline can be purchased anytime anywhere, and for trips of over about 400 miles you can't beat Southwest Airlines (especially if there's convenient transit at the airports - something we need to work on). The long-distance (typical runs of 500+ miles) Amtrak trains don't need speedup at all - they're cruise ships on rails and minimal accommodation for some rural stations and as long as their schedules can be made RELIABLE there's no reason for them to travel much faster than freight trains.