Around where I live, we frequently have trains running that are way way longer than some of the platforms at stations - my local station for example has space for at most 6 carriages when trains can be anything from 2 to 12 long. All the modern units have their own variant of SDO, selective door opening but they seem very overly complicated and/or inefficient.

Southern, operating Electrostar units, use a GPS system so the train knows where it is and releases the correct number of doors automatically (it's very slick), but has the problems that 1) if the system goes on the blink then it won't allow any doors to open, and 2) if the driver fails to stop in the correct place (ie doesn't go far enough along the platform) then trailing coaches may be opened with no platform alongside.

South West, using Desiro units, instead have the conductor move to the cab of the first unit that is not toally within the platform; releasing the doors from this cab only operates doors in front of the cab, meaning that in my aforementioned 6-coach station, only 4 will be available, and is also incredibly slow if the guard isn't on his toes.

So my idea is to put some form of indicator on the track in line with the platform area which the train can detect, and only doors that are above the indicator will release.

I pondered a few methods of doing this but many would be easily affected by things like snow. So, the option I think would work best would be an unpowered fourth rail, identical to the third rail power, but running in the middle of the track (like on the London Underground). Hardware identical to the conductor shoes could then be mounted on the coaches, under each door for maximum granularity. The shoe would either trip a mechanical switch when in contact with the rail (shoe rails slope down at either end so a shoe in contact is physically raised) or, if the fourth rail was electrically connected to the running rails, would be grounded, meaning it could be used for a direct electronic input.
-- K o R, Feb 26 2012

Platform screen doors http://en.wikipedia...atform_screen_doors
Not what this idea is about. [K o R, Feb 29 2012]

you need to be an athlete to get on or off trains at Clapham Junction. Grrrrr.
-- po, Feb 26 2012


po - Busiest railway station in Britain and yet it can't take 12-coach trains either!
-- K o R, Feb 26 2012


Why do anything ?

Just open all the doors. Some passengers will plummet two metres onto the ballast, and die.

Eventually (in a surprisingly short time) selection pressures will weed out those rail travellers who are too stupid to ckeck if there's actually a platform there before stepping off.

Thus the problem is resolved, at no cost.
-- 8th of 7, Feb 26 2012


//many would be easily affected by things like snow//

Actually when I read the title my brain immediately turned up a lever at the base of the door, that would be tripped when the platform brushed against it. But yeah, snow might trip it too if there's enough of it.

Bun for something that's needed, and that you seem to have thought out better than I did.
-- Psalm_97, Feb 26 2012


8th of 7: Natural selection is good, unfortunately Elfin Safety don't approve of its usage.

Psalm: That's a more HB approach, certainly, but it runs into difficulties on things like curved platforms (sometimes there's a gap of nearly a foot between the platform and the door edge!).
-- K o R, Feb 26 2012


//Psalm: That's a more HB approach, certainly, but...//

So... I'm in? I'm IN!!!

*Psalm_97 jumps up down excitedly
-- Psalm_97, Feb 26 2012


What's wrong with a simple laser or EM detector that determines whether the door is adjacent to a platform or just open air? It would be a bolt-on device, much less expensive and maintenance-intensive than a third or fourth rail with shoe.
-- Alterother, Feb 26 2012


[8th of 7], there are other pressures, not necessarily selective in the way you envisioned. Consider that in Japan they have guys on the platforms who have the job of squeezing as many extra passengers as they can, into the carriages, before the doors close. So, when the doors open again, at the next station, that pressure will want to be released. Those standing next to the doors may not have much choice, about exiting, regardless of whether they know a platform is present.

I was thinking of a simple sonar device interlinked with the door-opening mechanism. If such devices can be used to autofocus a camera (pioneered by Polaroid), then they should be able to notice whether a platform is present.
-- Vernon, Feb 26 2012


Sonar, even better than laser or EM! That way there won't have to be a reflective strip along the edge of the platform.

In the US, every rail car (passenger and freight) has an RFID tag installed for tracking purposes. The weatherproof casing is about 2" x 5" and mounts on a standardized bracket; the platform detector could easily be contained in a similar casing and mounted using the same style of bracket, which would significantly reduce manufacture/installation expense.
-- Alterother, Feb 26 2012


CJ is one of the busiest stations in europe for people with long legs and head for heights.
-- po, Feb 26 2012


// Those standing next to the doors may not have much choice, about exiting, regardless of whether they know a platform is present. // ... and will therefore be eliminated from the travelling population.

Therefore, the numbers of travellers will decrease.

Therefore, the trains will be less crowded.

Therefore, the "pushers" will no longer be needed, since the trains are less crowded. They can be made redundant.

Therefore, by doing nothing and allowing "market forces" to operate, there will be cost saving.

Q.E.D.

And your point is ... ?
-- 8th of 7, Feb 26 2012


[8th of 7], fewer passengers could cause the train service to go bankrupt. Then what?
-- Vernon, Feb 26 2012


Unlikely; the Japanese government just unloaded their passenger rail system onto the private sector, and I imagine they'd turn themselves inside out to avoid getting it back.
-- Alterother, Feb 26 2012


//fewer passengers could cause the train service to go bankrupt//

No, the force cited as picking these passengers off would diminish in almost logarithmic proportion to the number of passengers. And below a certain point this force would disappear completely.
-- Psalm_97, Feb 26 2012


// fewer passengers could cause the train service to go bankrupt. Then what? //

No, because the internal organs and other valuable parts of the "eliminated" passengers can be sold on the open market, providing a revenue stream somewhat greater than that ever delivered by said passenger's expenditure on travel tickets.
-- 8th of 7, Feb 26 2012


If the financial viability of the system hinges on the "overflow" passengers, then they have bigger problems than whether there is a platform.
-- methinksnot, Feb 27 2012


cut the trains in half.
-- simonj, Feb 27 2012


Zip-lines.
Individual zip-lines.

It's really the only answer.
-- 2 fries shy of a happy meal, Feb 27 2012


Alterother: You reckon a laser or EM detector is simpler to maintain than a switch that is mechanically tripped?

In all seriousness, the problem with using anything to detect the existing platform profile is that they are so variable. The idea of the rail is it's unintrusive, doesn't really require any maintenance (it's just a rail, nothing fancy about it) and works as a definite platform marker; likewise the shoes on the train are very simple: they drop by gravity if not over the rail, no springs, virtually no maintenance at all.
-- K o R, Feb 27 2012


On the St. Louis Metrolink, the geniuses in charge put up fencing that was designed to keep people from walking off of the platform into the gap between cars (who in the world did this, I have no idea, as the lack of a roof or anything resembling a door should have kept people out of the gap, but whatever).

Conductors then had a problem with pulling too far forward or not forward enough, meaning that the fencing blocked a door on the leading or trailing car.

So they just put up a guidepost. It's a simple vertical black plastic tube that tells the driver "This is where you stop." Because they're vertical, there's no problem with snow accumulation obstructing them.

tl;dr: just put up some simple reflective tape on posts or on the walls every so often at the end of a station, depending on carriage length. Driver stops once, releases batch 1 of communting horde, closes doors, eases forward, releases batch 2 of horde.

[+] for complexity.
-- shapu, Feb 27 2012


If the trains could be smart enough to know where they were, they could also probably figure out how to program the doors for each stop.
-- RayfordSteele, Feb 27 2012


// You reckon a laser or EM detector is simpler to maintain than a switch that is mechanically tripped? //

Yes, because an electronic detector contains no moving parts. In all the years I worked as a carknocker, the only parts of a box car that I never replaced were the center- sill (the big longitudinally-oriented frame component that serves as the 'backbone' of every type of rail car) and the tracking tag, and I'm not exaggerating. Every mechanical device needs routine servicing and replacement, especially on rail cars. Omniscient though I am not, I'd be hard-pressed to name an industry that abuses its equipment more than the railroad.
-- Alterother, Feb 27 2012


Worse: mining, particularly coal mining.

MUCH worse: The military
-- 8th of 7, Feb 27 2012


I didn't think of mining. I did think of the military, though I don't consider the Armed Services themselves to be an 'industry', but I discounted it because one of the primary functions of military equipment is to withstand abuse, both incidental and deliberate. Two good examples, though. The beating taken by railroad equipment is probably of a similar caliber, especially freight cars. They really get punished out there. I've refurbished cars that a layman would swear had been in a wreck, but the damage was simply accumulated by years of routine battering.
-- Alterother, Feb 27 2012


Yes, there are already markers on station platforms to tell the drivers where to stop. The device is to provide a much simpler means of automating which doors open without relying on the complex GPS system, or the inefficient limited door release. No matter where the train stops in the station, all doors that are alongside platform will operate.
-- K o R, Feb 27 2012


[+] Several parking garages in my area use (what I presume to be) a sonar device mounted over every parking space to detect if a car is in the spot, and turn on a red or green light accordingly. Why not use something like that? They're cheap, fairly reliable, and the devices and communication system should be pretty simple to install/retrofit.
-- ytk, Feb 27 2012


That might also be a pressure or magnetic sensor embedded in the floor, like those used at intersections with 'smart' signals. Could work in this application as well.
-- Alterother, Feb 28 2012


I imagined that the platform-detecting doors would be complemented by a platform-detecting giant boot. The passengers wanting to alight gather immediately inside a door; when the platform detector is triggered, the door and boot are triggered in quick succession, efficiently propelling the passengers into a giant catcher's mitt on the platform, thus eliminating wasteful stops.
-- spidermother, Feb 28 2012


The "simple system" this idea describes has been used for many years on elevators, hasn't it? The system doesn't change a great deal just because the cars travel horizontally... [ ]
-- Grogster, Feb 28 2012


Elevators work over a relatively short distance, are completely enclosed, and are integrated into the "track" in a way that trains cannot be. You can't have more than one elevator per shaft, elevators cars can't run in a continuous loop (paternosters excepted, of course), elevator cars are unable to switch shafts in mid-transit, and so on.
-- ytk, Feb 28 2012


...not exposed to the elements, not repeatedly bashed into each other, not perforated by fork-truck tines on a weekly basis...
-- Alterother, Feb 29 2012


Some (underground) train stations in Singapore have a tall barrier on the edge of the platform, fitted with sliding doors. When a train stops, its doors align with the platform's doors, and both open, allowing passengers to step through. I always assumed that sensors were involved, to prevent doors on either side from opening when not properly aligned, so this idea may be baked.
-- spidermother, Feb 29 2012


It seems to me that this whole problem is caused by the incorrect placement of doors on trains when, really, a means of egress is only required on the platforms (although I'll concede that there probably needs to be some sort of contingency for emergencies, perhaps a passenger ejection system). I'd go for a sliding roof section on the train which opens when stationary and some sort of articulated vacuum tube on the platform which can be lowered in through the gap to suck out passengers standing in the clearly marked & designated exit area.
-- DrBob, Feb 29 2012


Easier to have retracting floors, dumping the passengers into a basement chute.
-- pocmloc, Feb 29 2012


//Worse: mining, particularly coal mining.

MUCH worse: The military//
Far, far worse: Exhibits at childrens' science parks.
-- AbsintheWithoutLeave, Feb 29 2012


spidermother: This idea is quite different from platform screen doors. They are to stop people from falling onto the track; a consequence of their use is that they have to be able to detect when a train is alongside and only open sufficeint doors for the length of the train. This idea is to allow a train of any length to stop anywhere along a platform and only those doors that are alongside the platform will be openable.
-- K o R, Feb 29 2012


...which is a great idea and entirely possible, though the actual mechanism that should be employed remains a topic of heated debate.

Things like that make [The Alterother] happy.
-- Alterother, Feb 29 2012


I would use an industrial proximity detector beneath the door to detect the metal edge in the platform. Simple, reliable, tested.
-- haywardt, Feb 29 2012


haywardt: Unless the platforms aren't made of metal. Which ours generally are not (concrete and brick ftw. Wood in some cases - check out Berrylands some time).
-- K o R, Feb 29 2012


//This idea is quite different from platform screen doors.// Yes, but I was assuming that the train itself has sensors, so that only those train doors that are adjacent to a platform door open. If so, they would constitute platform-detecting train doors. I realise that your idea is not exactly the same. Sorry for not spelling that out.
-- spidermother, Feb 29 2012


// Unless the platforms aren't made of metal. //

An adhesive metallic tape running along under the lip of the platform would fix that. In fact, you could use the same kind of tape that's made specifically to trip industrial-use proximity sensors.
-- Alterother, Feb 29 2012



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