* Better track. Pretty obvious, huh?
* More power. Faster you go and the heavier you are, the more power you need. The more Kw per seat, the better.
* Better signalling. Most of the world tends to agree that not crashing is a Good Thing(tm). This generally meanms cab signals, automagic stops, and PTC / or modified signal rules.
* Above 125 or so - no freight on the same tracks, period. Freights tear up the track and beat it out of shape. Even an overweight train like an Acela is nothing Vs a freight.
* Light weight. Weight directly effects track maintenance and operation costs, period.
* Better suspension/trucks. Most HSTs have body mounted motors, special trucks, and more and more hollow axles. The trick is to get the speed at which the truck self oscilates (hunts) to be significantly higher than the max speed. Supposedly, the TGV has a safety margin of over 100mph without the use of dampers, here. The Acela has effectively a negative margin - without dampers it hunts even in the normal operational range.
* Electric traction. Almost a requisite, or you get killed on power costs and suffer bigtime on acceleration. Expect 12.5k or 25k at the wire although 15kv 16.3 hz and even 3,000 volt DC has been used on the TGV. Few diesels are light/powerful enough, turbines aren't efficient or low maintenance.
* Brakes! Multiple discs per axle and dynamic or regen. once again, heat increases with speed and weight.
* Steep grades. The things that make an HST go fast also mean it can climb a 4% grade with ease. Makes planning/building lines easy. But...
* Fewer curves. ...Curves are an issue even with tilt technology, which adds weight while also imposing even stricter weight limits. The highest unbalance used in the world is in Germany (11+inches!) where the track/train has to be specially certified and the loading is so critical the train is weighed en route. 7 - 9 inches is more practical, but demands high maintenance standards and good equipment (FWIW, the X-2000 ran it in service in the US. The Acela still doesn't and likely never will).
*Grade sepparation. Required above 100mph or so, but where speeds are lower, not required, but useful still.
* Precision maintenance. Using a hammer to 'make it fit' and ignoring the roadbed is bad bad bad. Once again, light weight drops these costs dramatically.