• what are these two things between the running rails

  • Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.
Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.

Moderators: mtuandrew, therock, Robert Paniagua

  by wrampart
 
The attached photo was taken at the King Street station.

First, what is the box with the white on green number 20?

Second, immediately to the right, there is a thin rail. I've seen this sometimes on over passes. Its a very thin rail than runs between the running rails. What is this for?

Thanks.
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  by Sand Box John
 
"wrampart"
The attached photo was taken at the King Street station.

First, what is the box with the white on green number 20?


That is a marker coil. It is used by the train board automatic control system to tell the train where the center of the station platform is and what the grade profile is. The marker coils are used to automatically execute a programmed station stop. The antenna that picks up the signals from the marker coils is located under the drawbar between the 1st and 2nd car of the train. There are a total of 14 marker coils for each station platform, 7 on each side of the center of the platform. The coil shown in the photo is 167' from the center of the platform. the other six, in pairs, are located 484', 1,200' and 2,700' from the center of the platform. One of the marker coils in the pairs at 484' and 1,200' are variable frequency marker coils. The variable frequency marker coils can be used to tell trains that are less then 8 cars long to stop short or long from the center of the platform.

Precision Station Stopping Progress Update (6.69 MB PDF file) pages 4-6 and 10.

The only station where the variable frequency marker coils are used is in Gallery Place on the Red Line platforms. Less then 8 cars train to Shady Grove stop short, Less then 8 cars train to Glenmont stop long.

The purpose of the short, long stop function is to stop trains that are less then 8 cars at end of the platform where most of the passengers are located.

At one time short, long stop controls were located in the major kiosk of every station in the system. They were removed from the station managers kiosk to eliminate their maintenance because they were rarely used.

Second, immediately to the right, there is a thin rail. I've seen this sometimes on over passes. Its a very thin rail than runs between the running rails. What is this for?

Guardrail to prevent derailed train from falling off elevated structures and overpasses.
Last edited by Sand Box John on Sun Aug 16, 2009 7:27 am, edited 2 times in total.
  by justalurker66
 
wrampart wrote:Great. Thanks for the detailed response (at least for the first question :)
There isn't much more to say about the second part. :)
Those inner guard rails are common on bridges and elevated structures even off WMATA ... just a little help to stop a train that jumps the outside rail from going over the edge.
  by wrampart
 
I thought those were only on curved sections of track. Traveling through East Falls Church (straight, on grade track), I also so one of these rails. Its not particularly close to the running rails like I've seen on curved section of track (especially the tight curves going into yards).

Thanks!
  by Sand Box John
 
"wrampart"
I thought those were only on curved sections of track. Traveling through East Falls Church (straight, on grade track), I also so one of these rails. Its not particularly close to the running rails like I've seen on curved section of track (especially the tight curves going into yards).


You are talking about 2 different verities of guardrails. The angle iron guardrail seen in your picture above is as I remarked above "to prevent derailed train from falling off elevated structures and overpasses."

The guardrails seen in the turnout and your yard example below are there to keep the wheel flanges from climbing up the outside rail of the curve.

Image

North end of National Airport by James Calder 02 18 2006.


Image

New Carrollton Yard Loop 04 16 2006.
  by farecard
 
Sand Box John wrote:The antenna that picks up the signals from the marker coils is located under the drawbar between the 1st and 2nd car of the train.
I'd use slightly different wording. It's really analogous to a turning fork. Each marker contain a tuned L-C circuit of a specific frequency. The antenna on the train excites the circuit, and the circuit resonates. The train's electronics detects which frequency the marker is resonating at, and thus knows where in the stopping roadmap it is. I believe that different frequencies control the right side vs left side door opening....or DID, when WMATA used automagic door control, and may again.

The "doghouses" are thus passive; they need no trackbed power wiring or active electronics. [Sometimes you'll see a spare/old ones piled outside a station.] That makes them more dependable and easy to replace as needed.
  by Sand Box John
 
"Sand Box John"
The antenna that picks up the signals from the marker coils is located under the drawbar between the 1st and 2nd car of the train.


"farecard"
I'd use slightly different wording. It's really analogous to a turning fork. Each marker contain a tuned L-C circuit of a specific frequency. The antenna on the train excites the circuit, and the circuit resonates. The train's electronics detects which frequency the marker is resonating at, and thus knows where in the stopping roadmap it is. I believe that different frequencies control the right side vs left side door opening....or DID, when WMATA used automagic door control, and may again.

The "doghouses" are thus passive; they need no trackbed power wiring or active electronics. [Sometimes you'll see a spare/old ones piled outside a station.] That makes them more dependable and easy to replace as needed.


You are quite correct. I used that wording for the propose of simplicity.

The two variable frequency marker coils do in fact have wiring that goes back to the train control room, the other five do not.
  by farecard
 
The only station where the variable frequency marker coils are used is in Gallery Place on the Red Line platforms. Less then 8 cars train to Shady Grove stop short, Less then 8 cars train to Glenmont stop long.
Do I infer this to mean they change modes when running backwards for Christmas [1] err single-tracking?

Isn't there another variable at every station, the train length? By that I mean when they were attempting center-of-the-platform stopping, a longer train has to go further past the doghouse than a shorter one. I assume the on-board system is what accomplishes err attempts that goal..

BTW, I wonder iffen they will move the doghouses to permanently make the stopping point the end of the platform?






1] Obscure old Goon Show reference...
  by Sand Box John
 
"Sand Box John"
The only station where the variable frequency marker coils are used is in Gallery Place on the Red Line platforms. Less then 8 cars train to Shady Grove stop short, Less then 8 cars train to Glenmont stop long.


"farecard"
Do I infer this to mean they change modes when running backwards for Christmas [1] err single-tracking?


No. Like I said in my description above there are 14 marker coils for each platform, 7 on either side of the center of the platform. 7 marker coils for normal direction of travel, 7 marker coils for reverse direction of travel.

Isn't there another variable at every station, the train length? By that I mean when they were attempting center-of-the-platform stopping, a longer train has to go further past the doghouse than a shorter one. I assume the on-board system is what accomplishes err attempts that goal..

The train knows how long it is. The marker coil tells the train where the center of the platform is. The train knows how far it travels based on the distance it travels by counting the revolutions of the wheels. The trains onboard hardware processes that information to determine the correct stopping location.

BTW, I wonder iffen they will move the doghouses to permanently make the stopping point the end of the platform?

Won't work. The onboard hardware and software is configured for marker coils located at 167', 484', 1,200' and 2,700' from the center of the platform. If you move the marker coils beyond the center of the platform to normal stop a 6 car trains at the end of the platform, 8 car trains will over shoot the platform by 75'.

Though not really applicable to today's operating conditions. Train less then 6 cars long do not stop at the end of the platform when programmed to stop short or long. 4 car trains will stop 75' from the end of platform when stopping long, 4 car trains will stop 225' from the end of platform when stopping short.
2 car train will stop 150' from the end of platform when stopping long, 2 car trains will stop 300' from the end of platform when stopping short.
  by Sand Box John
 
"Sand Box John"
The only station where the variable frequency marker coils are used is in Gallery Place on the Red Line platforms. Less then 8 cars train to Shady Grove stop short, Less then 8 cars train to Glenmont stop long.


"farecard"
Do I infer this to mean they change modes when running backwards for Christmas [1] err single-tracking?


No. Like I said in my description above there are 14 marker coils for each platform, 7 on either side of the center of the platform. 7 marker coils for normal direction of travel, 7 marker coils for reverse direction of travel.

Isn't there another variable at every station, the train length? By that I mean when they were attempting center-of-the-platform stopping, a longer train has to go further past the doghouse than a shorter one. I assume the on-board system is what accomplishes err attempts that goal..

The train knows how long it is. The marker coil tells the train where the center of the platform is. The train knows how far it travels based on the distance it travels by counting the revolutions of the wheels. The trains onboard hardware processes that information to determine the correct stopping location.

BTW, I wonder iffen they will move the doghouses to permanently make the stopping point the end of the platform?

Won't work. The onboard hardware and software is configured for marker coils located at 167', 484', 1,200' and 2,700' from the center of the platform. If you move the marker coils beyond the center of the platform to normal stop a 6 car trains at the end of the platform, 8 car trains will over shoot the platform by 75'.

Though not really applicable to today's operating conditions. Trains less then 6 cars long do not stop at the end of the platform when programmed to stop short or long. 4 car trains will stop 75' from the end of platform when stopping long, 4 car trains will stop 225' from the end of platform when stopping short.
2 car train will stop 150' from the end of platform when stopping long, 2 car trains will stop 300' from the end of platform when stopping short.
  by justalurker66
 
It seems that it would help to think of them as location markers (to find the center of a platform) and not "stop here" signs.