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Comment by melling | Hacker News Reader

What’s the average speed?

For a distance of 30 miles, shouldnt we be able to build a train that averages 60 mph for 30 minutes?

China is working on low-speed maglevs.

The problem is the rule of diminishing returns. Unless you want your passengers vomiting everywhere, there are limits to how quickly you can accelerate and decelerate, and you spend less and less time going at the top speed. It's why subways don't generally bother going much above a 50-70MPH top speed.

Unless you're building a straight line, you need larger and larger curves to accommodate high speed traffic, which makes the cost of higher speed rise significantly since you can't just buy a curve and split a property in two. A train going at 220 km/h has a minimum curve radius of 2.5km; a train at 300 km/h has a minimum curve radius of 4km; and a train at 350 km/h has a minimum curve radius of 7km.

What about a train going 120 kph (75mph)?

No one said you need to go 300 kph.

What’s an acceptable time to accelerate to that speed?

The train already has a top speed of 79mph. Anything further requires new investments in signalling and whatnot according to American regulations, because that tends to be the speed at which wayside signalling becomes hard to see.

A lot of trains have a high top speed yet average under 50 mph. What’s it take to get the average speed to 120 kph, for example?

> The problem is the rule of diminishing returns. Unless you want your passengers vomiting everywhere, there are limits to how quickly you can accelerate and decelerate

This thing has 30mn legs (Miami to Ft Lauredale and Ft Lauredale and West Palm Beach have no intermediate stops), it's not a commuter rail. Subways have a few minutes (and km) between stations.

French regional trains run this kind of route in under 20mn: https://www.google.fr/maps/dir/Dijon+Gare,+Dijon,+France/Bea...

And from experience I can tell you that they can be used comfortably standing, they accelerate no faster than a subway.

Your numbers seem off. Traveling in a circle at constant speed implies acceleration of v^2 / r, so, holding acceleration constant and doubling velocity, I'd expect the necessary radius to double. A Wiki article seems to roughly agree with me.

https://en.wikipedia.org/wiki/Minimum_railway_curve_radius#S...

However, from your numbers, 4km * (350 / 300)^2 = 5.4 km, not 7 km. The Wiki article agrees with the 300 km/h => 4km radius data point, but agrees with me for 350 km/h, and actually says 400 km/h => 7 km. Was it a typo on your part, or am I missing something?

I got those figures from here: https://books.google.com/books?id=apKZDgAAQBAJ&pg=PA187&lpg=.... But maybe the French are overengineering? Entirely possible.

I see, thanks for the citation. It looks like those are numbers for a few actual railways—i.e. "On railway X, the maximum speed somewhere along the track is n, and the minimum radius of curvature is r." Explanations could be: (a) the path of the train didn't really need any sharper turns (i.e. "minimum allowable radius" ≠ "minimum radius we chose"); (b) different architects, working at different times, possibly using different materials, chose different safety margins; (c) perhaps one of the lines is designed to carry taller or heavier trains.

I do believe that, holding everything else constant, the minimum allowable radius would be proportional to the square of the velocity of the train. If you look at table 6.15 on page 183, it compares 250 km/h with 350 km/h, and 350/250 = 7/5, and (7/5)^2 = 49/25 = 1.96, so I would predict the radius would be roughly 2x for 350 as for 250, and, if you compare corresponding entries in the table, you see this holds very well for the columns labeled "(1)" and for the rightmost three columns, although those labeled "(2)" (apparently done with a different calculation—I can't see the page the calculation is from) diverge somewhat.

The full Caltrain line is around 77 miles long.

Just the San Jose <-> San Francisco portion is ~47 miles.

Caltrain's timetables give the following for San Jose Diridon Station to San Francisco 4th and King Station, weekday service:

* Train 101, local, 20 stops, 04:28 -- 06:03 (1 hour 35 minutes)

* Train 207, local south half / limited north half, 12 stops, 05:59 -- 07:24 (1 hour 25 minutes)

* Train 211, limited south half / local north half, 17 stops, 06:23 -- 07:57 (1 hour 34 minutes)

* Train 215, limited, 9 stops, 06:54 -- 08:07 (1 hour 13 minutes)

* Train 305, "Baby Bullet" pattern A, 4 stops, 05:45 -- 06:47 (1 hour 2 minutes)

* Train 309, "Baby Bullet" pattern B, 5 stops, 06:04 -- 07:08 (1 hour 4 minutes)

So the best the system manages (the 4-stop "Baby Bullet" pattern) is 62 minutes for 47 miles, averaging just over 45mph. The worst (the local making all stops) is 95 minutes for 47 miles, averaging just under 30mph.

Factoring out the time spent stopped at the stations is unfortunately not possible from Caltrain's timetables, which only give the scheduled time of departure from each station.

The maglev in Changsha between the airport and HSR station runs at 100km/hour. It is on a viaduct for much of its path, however, and isn’t very urban (goes nowhere near downtown Changsha). It only averages 57km with stops.

https://en.m.wikipedia.org/wiki/Changsha_Maglev_Express

Having ridden on the Changsha Maglev I wouldn't suggest its a very well refined design. Lots of shuddering and certainly didn't seem designed for anything much more than commuter speed.

An other commenter above quoted ~50mph average for the inaugural run.

> For a distance of 30 miles, shouldnt we be able to build a train that averages 60 mph for 30 minutes?

Euro regional rail does 80+mph for this sort of non-stop distances.

Its possible to achieve substantially higher speeds, but that would require grade separation and electrification, which are much too expensive for a private company to take on.

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