[chrismartin]

Has anyone considered the following? In a small town, you have a section of track running parallel to high speed rail. The track has a small and short "local" train (maybe just a couple of cars) that picks up passengers and accelerates to maybe 80 MPH, while the high-speed train slows to the same speed. The trains run next to each other for a couple of miles, some doors open between them, and people can step between the "local" train and the "long distance" train.

This lets the high-speed train serve a lot more places without losing much speed. Maybe the local train serves several towns in the area.

[bobthepanda]

It has been considered and basically it’s not really reliable or practical or safe.

[black_puppydog]

It was also planned for cargo (e.g. at the Megahub Lehrte) but didn't really pan out there either, even though ISO containers are much more predictable in their self-propelled movement than humans. At least all the material from that site now shows much automation, but it all happens at rest.

[bombcar]

The problem is it's doable but if you do it, you might as well have a slow, regional railroad that goes between the stops of the fast international railroad. So a local/express situation, which is much simpler technology-wise.

[FVYPblNGl7R9ZAc]

Is there any writings on this? It sounds interesting.

[bobthepanda]

My google fu is failing me but here are a few of the problems.

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Reliability

For a connection like this to work the trains have to meet, consistently, every hour or however frequently they run. This is pretty difficult, most rail networks are highly complex and delays cascade across a network. With station transfers this is okay, you can drop off anyone who needs to transfer to wait and let everyone else continue, but in this sort of moving transfer either people just totally miss their connection because it wasn't there, or the train loops around somehow and delays everyone else continuing on.

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Practicality

Let's say trains meet at 80 mph. Assuming you give people one minute to get their bags, walk to the door, take their seats in the other train (fairly aggressive for anyone who isn't a fit young adult) that means you spend 1.3 miles of distance traveling. That means 1.3 miles of parallel track where neither train can actually stop to serve local communities.

Station transfers in comparison are fairly compact (the length of a train) and they can actually also let people on and off from the surrounding areas with appropriate exits, whereas that's not possible with moving trains.

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Safety

We can't really guarantee two trains will move at the same speed parallel to each other. Trains are not automated to such a degree outside of self-contained metro networks, full automation is too complex to do in one shot and partial automation is still very complex and in its early days. Also whatever physical mechanism has to be tight enough to accommodate accessibility (wheelchairs can't go over large gaps) and reliable enough to work all the time; what happens if said mechanism breaks down while the trains are moving and conjoined?

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Finally there's the matter of actual need. If you need trains to not get slowed down, you can have some express services skip connecting to local regional services at all. It turns out demand for that kind of service is fairly limited; as an example, Amtrak has tried several times to introduce nonstop DC to NYC service, but it turns out that the additional passengers do not offset the loss of passengers from Baltimore, Wilmington and Philadelphia.

[samatman]

I've mused about a similar idea for California's high speed rail, which, should it ever be built, would be rendered impractically slow by frequent stops.

The idea is to drop cars without slowing down. These cars would have brakes, that's it. Before the station, drop the car, it slows enough to give safe time for switching, and cruises to a halt at the local station.

That's drop off, pickup is the slow train, which runs twice a day in each direction and assembles the carriage on the way.

Impractical for various reasons, sure. But what if.

[not2b]

The plan for California high speed rail was to replace plane flights and long distance car travel, not to have frequent stops (at least 40 miles/65 km between stops, sometimes longer). Only the major cities.

[samatman]

Yes, that was the plan. The raison d'être, even. But if it ever gets built, it will be obliged for political reasons to stop so often as to make it an unattractive alternative to flying from SF to LA or vice versa.

[soneil]

I could swear I've seen illustrations of such concepts. In the same theme as circular runways and having runways balanced across the tops of city skyscrapers.

The "moving platforms" concept seems to come up quite often, ala https://www.youtube.com/watch?v=MlhuHmVn4Ss