In our last exciting episode we visited Trantor, or at least a city world that could pass for Trantor, an ecumenopolis of a trillion people for whom the Galactic Empire is something of an afterthought. But having come this far we ought to see more of it than just the inside of a spaceport gate or elevator foyer.
The Foundation 'verse has no teleportation tech (if you exclude jump-style hyperspace), so to get anywhere on a planet you have to go there. And if our ecumenopolis is a real city - not a mere planetwide suburbia (how boring!) - this means a public transit network.
Cars will not do, not for general use - not even Futuristic[TM] skimmer cars or whatever. Not even robocabs, which did not exist in that 'verse anyway.
It is all about geometry. Cars and highways are fine at low population density, such as tract-home suburbs - say, 5000 people per square mile = 2000/km2. But once you get up even to townhouse urban density, about 10,000/mi2 = 4000/km2, parking and roadway space become a major hassle.
Putting everyone in robocabs instead of private cars would help with parking, and the robo part would allow more roadway crowding before gridlock sets in. (Robots should drive better than we do.) Boxy little cars would help a bit, too. But these measures only get you so far.
Yes, in principle you could have multiple layers of parking garages and underground roadways below the dwellings. And since Asimov's 'verse did have aircars, you could move the garages to the top floors and the traffic jams into the air. In the Future, car accidents never happen.
But in practice, at some point it gets easier and faster to simply take the bus.
Which brings us to transit technology. In rocketpunk days it was taken for granted that even ground vehicle would never use anything so primitive as wheels. As late as c. 1990, the agency building the Los Angeles rail transit lines insisted that artists' conceptions avoid showing that its trains would run on (gasp!) railroad tracks.
Now, of course, tracks are back, including streetcars (trams, to some of you). Depending on the state of the tracks or pavement, streetcars usually have a smoother ride than buses, but don't go any faster. Their chief virtue is that a streetcar, running on rails, can be longer than a bus and thus carry more people.
But this only really matters for very busy lines, which is why most streetcars vanished around the rocketpunk era.* In any case, by the near future - no need to wait for the plausible midfuture, let alone the Galactic Era - technology could blur these distinctions.
The rocketpunk era is associated less with fading streetcars than with two other forms of urban transit. One, monorails, needs little discussion here. They are just elevated (usually) rail lines with a track too narrow for most idiots to try walking along. (The wheels are also neatly hidden from view.)
Much loved in the abstract, monorails never became popular in real life because people hate els running above their street. This is too bad, because you see a lot more of the city from an el than you do from a subway. But most people hate on els anyway, and still hate them even when the tracks are narrower and don't blot out quite as much sun. Which is why monorails remain rare.
The other great rocketpunk transit tech was the slidewalk, a pedestrian conveyor belt resembling a flattened out escalator. Step on and be carried along. These are more interesting, as a real departure from conventional vehicular transit. For one thing, slidewalks run continually, so unlike a bus or train you don't have to wait for it. This is a big deal, because people hate waiting, and long wait times can effectively wipe out the advantage of high speed.
Like monorails, some slidewalks actually exist, but also like monorails they have never really caught on. The problem is that if they are fast enough to save you much time over walking, people will stumble and fall all over when getting on or off.
Heinlein (and probably others) suggested multiple side-by-side strips, so you could start on a slow 'local' strip, then cross over to faster express lanes. Alas, unless Trantor has UBT - universal ballet training - this side-step across a speed differential is also a guaranteed pratfall generator.
With suitable magitech you might improve on the situation. Clarke's far future city of Diaspar has slidewalks made of flowing 'anisotropic matter'** that you can stand/walk on, while allowing a smooth transition from slow edges to the faster center express section.
Slidewalks are still limited in speed unless your magitech also moves the ambient air along so riders aren't facing a gale-force relative wind. Rapid transit they are not, but if you can solve the pratfall problem they might have a place along busy corridors like Seldon Street.
Indeed, for window shoppers and flaneurs, Seldon Street might even have slidewalk cafes and such. But this is more
Otherwise your basic local line, the service that goes everywhere and stops at your corner, is essentially a plain old bus. Even though the smelly diesel bus has surely gone the way of the 19th century horsecar, which also emitted a noxious exhaust.
And so a bus it is, though heavy Trantorian ridership levels - especially along busy Seldon Street - might justify streetcars/trams. We will also suppose that heavy ridership allows TERTA to provide frequent service, so you only have a short wait when connecting between lines.
But the local bus can't be very rapid, not for techological reasons as such, but because it has to fight its way through traffic, automotive or pedestrian. Even if separated from other traffic, it must stop every few blocks to let riders on and off. And it can't get up too much speed between stops because of a basic human limit.
Back in the 1930s, the R&D program for the classic American PCC streetcar determined that the highest comfortable acceleration for transit straphangers is about 0.2 g, or two meters/second^2. The maximum acceleration of the PCC was thus set close to this level - quaintly expressed as 4.75 mphps (miles per hour per second).
For surface vehicles that 'push against' the road or track, power needed for a given acceleration rises with the square of velocity; to avoid wasteful design, average acceleration for all but the most local service will be about half the maximum, a nice even one meter/second^2 or 0.1 g.
Absent magitech pseudo-gravity to allow high acceleration without bowling passengers over, technology cannot dramatically change these constraints, which is why present-day transit lines are not much faster than those of 100 years ago.
Between the acceleration limit and the need for frequent stops (with 'dwell time' for riders getting on or off), the average or service speed of local transit is limited to about 15 mph / 25 kmh or thereabouts. Fighting through traffic makes it a good deal slower, unless the the line runs on its own reserved speedway in a boulevard median - an arrangement both useful and rather elegant.
To get around this practical speed limit, large present-day cities have a two-level transit heirarchy. The local bus runs everywhere. Layered above it - or more often below it, in a subway - is a rapid transit or metro system, unimpeded by other traffic, with lines and stations more widely spaced, typically in the range of a kilometer to a mile apart.
Because the rapid transit trunk lines have heavy ridership they are commonly served by multi-car trains, not individual buses. So we will simply call the rapid transit vehicles trains, without further ado.
Longer runs between stations allow higher top speed for the same acceleration, and rapid transit service speeds are in the range of 25 mph / 40 kmh.
A two layer transit hierarchy is enough for most present day cities. Paris has a third, the RER, and London is developing one, the London Overground, upgrading and connecting suburban commuter lines for frequent all day service.
This heirarchy is not rigid - 'light rail' and 'bus rapid transit' both tend to be intermediate or hybrid cases - but it provides a starting point for discussion. And as commenters on the last post already anticipated, Trantor will need multiple layers in its transit heirarchy. Just how many is hard to say; we don't have even semi-ecumenopolitan examples to guide us.
We can start by considering a performance level that is not remotely magitech. Suppose a train accelerates at an average 0.1 g to a maximum 150 m/s, about 330 mph, then decelerates at the same rate - the public transit equivalent of a brachistochrone orbit. (This only loosely resembles how rail vehicles move, but gives us a first approximation.)
Travel time is about 300 seconds, five minutes, and the vehicle goes 22.5 km. If we let the train cruise at top speed for another 150 seconds, we go 45 km in seven and a half minutes. An express run, passing intermediate stations, can go nearly 160 km - 100 miles - in 20 minutes.
For comparison, the fastest existing transit line, the Shanghai airport maglev, runs 30.5 km in 7:20, hitting a top speed of 120 m/s. So the model performance is only modestly above current rail practice. These high speed lines can form the third layer of the rapid transit hierarchy. Below them, heirarchically, are regional trains that stop every few miles or km, then the primary metro subway (and finally the local bus).
Allowing time to get from your home to the high speed rail station, and from the destination station to wherever you're actually going, this type of system - local bus plus a 3-layer hierarchy of rapid transit, will get you pretty much anywhere in the extended neighborhood within an hour, where the extended neighborhood extends a hundred miles or so.
At typically modest Trantorian urban density, up to half a billion people live within this radius (fewer if there are geographical constraints like a coastline, large park, or the Imperial Palace grounds.) So within an hour's ride are a corresponding number and variety of jobs, restaurants, potential lovers, and whatever else the city has to offer.
But to really get around town we need to go faster. Suppose now a one hour 'semi brachistochrone' - 20 minutes accelerating at 0.1 g, 20 minutes cruise, 20 minutes decelerating. This takes you nearly 2900 km, 1800 miles, about the length of Seldon Street. Top speed is 1.2 km/s: rapid transit, indeed!
The currently popular technology for this type of service is a hyperloop. Unfortunately, in current proposals the accent is on hype - not because the tech is modestly speculative, but because promoters tend to shamelessly lo-ball things that are not speculative at all, such as the cost of building elevated viaducts.
(The name hyperloop is unfortunate in another way; it sounds more like an Awesome roller coaster ride than a practical transit service for people who may be package-laden, tired, tipsy, or all three.)
But all that said, some such technology should be viable - essentially a genteel cousin of a mass driver or coilgun - and TERTA knows how to estimate construction costs. With a nod to London, the Mother of Rapid Transit, I will simply call these Tube lines.
For the longest trips, a two hour nonstop Tube takes you up to 11,500 km / 7000 miles, nearly a third of the way around an Earth sized planet. Allowing for all connections and wait times, you can get from most locations on Trantor to most other locations in perhaps five or six hours.
A 'local' Tube running 30 minutes between stops will go a quarter as far as the baseline model, around 700 km / 400 miles. This service thus runs the length of Seldon Street in two hours, with three intermediate stations.
Conventional high speed express trains connect these stations in turn, with another four or so intermediate stops, and so on down the hierarchy to the primary rapid transit that stops every mile or less. Then there are the buses and streetcars, and perhaps slidewalks, along Seldon Street itself.
Thus a two hour trip - about the maximum for casual daily travel, whether commuting to work or meeting a friend for lunch - will get you more or less anywhere within a thousand km / 600 miles. If your destination lies close to a major transit hub you can go two or three times as far, because it will be served by top level lines, and you won't need to work your way back down the hierarchy.
The cityscape will reflect the granulation and heirarchy of the transit system. Most rapid transit stations will be nuclei of urban villages, neighborhood centers for errands, entertainment, and general public social life. Major stations will draw larger and denser condensations of the world city, some perhaps on a scale that would match our grandest Zeerust visions of the urban Future.
And while Trantor falls short of being a single practical commute zone, something in the range of 10-50 billion people probably live within two hours of wherever you are. Long distance travel might be constrained by high fares, but perhaps TERTA runs like the semilegendary subway of Gotham on the Origin World: a nickel takes you all over town.
Although not part of the urban transit system, a word about space elevators. I have argued that they are only suited to truly enormous volumes of space traffic. Well, here we are: If any world has the requirement, Trantor does. We can imagine numerous elevator lines rising from the equator, probably with ring lines connecting them at geosynch level. Commenter Eth noted last post that the elevator cables could also support a ring of solar collectors or radiators if needed for power or heat management.
Enough about the elevators; back to TERTA.
The system is extensive, with bus and rail lines totalling hundreds of millions of miles, served by up to a couple of billion buses and subway cars. The Tube network, serving only long haul trunk routes is a mere million miles or so, interconnecting perhaps a thousand stations - few enough that dedicated enthusiasts will have visited all of them.
These major stations should be suitably impressive. The levels of the transit hierarchy must become literal here, the long-haul Tube lines probably running deepest, with local lines being closer to the concourse and street entries. As with major airports today, a transit system might be needed simply to get around the station itself.
And at times it will seem as if all those trillion Trantorians are trying to catch the same train that you are.
But from suitable locations you can look down along some of the lines, with their diverging and converging switching networks and crossovers. The utter coolness of which is justification enough for this visit to Trantor.
* The Great Streetcar Conspiracy was real, but played only a minor role in their demise. Streetcars were unfashionable in the 1950s, and most systems were old and badly run down. So it was simpler to bus convert even the few lines busy enough that streetcar modernization would have been preferable.
** Anisotropic matter is also a term in relativity and cosmology, but I have no idea how it relates to the stuff you would use for slidewalks.
The image of the Second Avenue Subway comes from the New York City transit agency. Because even tiny villages like NYC can benefit from rapid transit!
The eastside Manhattan line, first proposed about a hundred years ago, opened on New Year's Day, and cost about $5 billion for a couple of miles of line - outrageously expensive even for subway lines, which are never cheap. But even at train robbery prices it will be worth the wait for the good citizens of Gotham.
And as a curious example of Google time lag, there are not yet any good post-opening images of the line, which explains the odd absence of New Yorkers on the station platform.
"To All Trains" is from the NY Transit Museum.