As you increase speed, you need to take turns at larger and larger curves to reduce g-forces on passengers, and the curve radius increases exponentially vs speed. The bigger the curve, the more land you need to buy, and depending on which country you're in property rights extend above and below the land so the cost is unavoidable. (If you can avoid it by going underground or overground, the cost of tunneling and bridges is expensive.) It becomes increasingly costly to increase speed, and on top of that you get diminishing returns with travel time (speeding up from 80 to 120MPH is worth a lot more than going from 200MPH to 240MPH). And above a certain point you spend too much time accelerating and decelerating from top speed, and not enough time actually at the top speed to make it all worthwhile.

Regardless of motive technology, curve-radius will always be a relevant problem, because one of the major advantages of the train is that you can walk around and about. We could design trains with sharper corners and harder acceleration/deceleration, but no one wants to be strapped into a vomit comet for three hours.

> and the curve radius increases exponentially vs speed.

It's merely quadratic if I recall correctly

Oops, I misspoke. But yeah, it's a non-linear increase in costs that results in increasingly marginal time savings.

Because it's here today. It's easy with simple investment, it's got huge benefits over the joke that is the current U.S train service.

There are plenty of passenger trains in service around the world that go faster than 200mp/h, today.

Those are all in China. Wikipedia tells me that max achievable speed in regular service in every country except China is 320km/h. China does 350km/h, but you need to be traveling at quite a long distance for a measly 30km/h increase in speed to be worth it, and China runs much longer high speed rail services than other countries (Beijing to Hong Kong is 9 hours).

Besides, the more important thing is average speed. Acela's top speed is respectable for legacy rail, but it achieves this top speed for a total of 33 miles over a 457 mile route.

> Those are all in China

You're probably right. America shouldn't aim to achieve speeds of high speed rail in ~2025 that China achieved in ~2015. That's aiming too high.

Best to aim lower so we don't disappoint anyone.

The most painful part of high-speed-rail, particularly at higher speeds, is the land acquisition costs; minimum curve radius grows exponentially as speed rises, so costs escalate as speed gets higher. China cares far less about compensating property owners for fair value, which is why they can engineer such high speed lines in the first place.

We are talking about the difference between 217MPH and 200MPH. The total travel time saved between Boston and Washington by an additional 17MPH is a whopping six minutes with the generous assumption that you start right off the bat at top speed and maintain it the entire trip. If saving six minutes will cost billions or tens of billions of dollars, why bother with such a marginal improvement, when you could spend those billions on literally anything else from housing to healthcare to upgrading the power grid?

And China pretty much stopped running train at 350km/h.

Steel-on-steel rail is realistically limited to top service speed of 320km/h, maybe 360 at most. The maintenance is just too high when you increase speed beyond that. Newer Japanese Shinkansen line only operate at 260km/h, for example.

Yes, but the US just needs a taste to get going. They don't need to aim too high, they should concentrate on delivering something.