Destination codes and train control systems

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Destination codes and train control systems

Postby drwho9437 » Sat Jul 12, 2014 2:18 pm

I have read the post of the destination codes of the trains in the WMATA rail system as well as looked at the system maps.

I have also read parts of the NTSB crash report near Takoma. I am curious basically how these codes are transmitted back and forth between the trains and the control systems.

The first question is, are the codes and train identifiers available outside of the WMATA's systems? They have a API for software development (which is basically what I am playing around with) but this uses the line identifiers and does not assign train numbers to the objects in the data stream.

If you read the NTSB report the control system function is at once both robust and crazy. It creates and destroys trains based on track block signal occupancy. On the one hand that ought to cover all contingencies, on the other hand it isn't a good physical model of the system. Trains can only enter the system at a very finite number of yards, at each block in the system they are topologically limited as to where they can go, trains cannot just disappear. My reading of the crash report suggests that the crash could have been prevented simply by a better software control system, because a real train was deleted by the control system.

My second question is how does the train "talk" to the control system?

My impression from the report is that almost all (all?) communications to the train take place via coded audio frequency transmissions between the two non-electrified rails. There was some mention of tuned coils as well. This would include maximum speed commands, the destination codes, and if the train should wait for schedule adjustment.

If anyone knows about these sort of things it will help me better understand how the system is controlled which will help me to model how it works in software.
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Re: Destination codes and train control systems

Postby drwho9437 » Sun Jul 13, 2014 5:47 pm

I think I may understand better now, after looking through Alstom's portfolio. It seems as though the tracks are segmented by impedance matching different sections of the track with blobs of passive parts which are referred to as impedance bonds. The signaling to the train however would appear to be inductive rather than conductive on very low carrier frequencies.
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Re: Destination codes and train control systems

Postby Sand Box John » Sun Jul 13, 2014 9:37 pm

"drwho9437"
I have read the post of the destination codes of the trains in the WMATA rail system as well as looked at the system maps.

I have also read parts of the NTSB crash report near Takoma. I am curious basically how these codes are transmitted back and forth between the trains and the control systems.

The first question is, are the codes and train identifiers available outside of the WMATA's systems?


No

They have a API for software development (which is basically what I am playing around with) but this uses the line identifiers and does not assign train numbers to the objects in the data stream.

If you read the NTSB report the control system function is at once both robust and crazy. It creates and destroys trains based on track block signal occupancy. On the one hand that ought to cover all contingencies, on the other hand it isn't a good physical model of the system. Trains can only enter the system at a very finite number of yards, at each block in the system they are topologically limited as to where they can go, trains cannot just disappear. My reading of the crash report suggests that the crash could have been prevented simply by a better software control system, because a real train was deleted by the control system.


In the case of the Fort Totten wreck the train disappeared because of malfunctioning hardware not software. Said hardware malfunctioned because it was not properly celebrated for the environment it was installed at, One end of the track circuit also preformed the function of traction power current return.

My second question is how does the train "talk" to the control system?

Train to wayside communication is transmitted through the rails at a frequency above the 8 track circuit frequency. A loop antenna mounted under the front of the lead car and under the rear end of the last car is used to transmit data to way side. The same antenna is used to receive speed commands for train control. The train continuously transmits a short massage containing the train destination and waits 100 milliseconds for a reply from wayside before repeating the the short massage The following data is transmitted to wayside from the train; train destination, train number, passenger station check, train ready, all doors closed, train berthed, motion detection and train length. Station check, train ready, all doors closed, train berthed and motion detection are used in the platform track circuit during the execution of a station stop.

My impression from the report is that almost all (all?) communications to the train take place via coded audio frequency transmissions between the two non-electrified rails.

That is correct.

There was some mention of tuned coils as well.

The tuned coils (marker coils) transmit grade profile and distance from center of platform information. They tell the train where it is in relationship to the center of the platform to execute automatic station stops. The antenna that receive the signal from the marker coils is mounted between the first and second car of the train. there are 7 marker coils on either side of the center of the platform. they are located at 150' , 484', 1200' and 2700' from the center of the platform. The 150' coil is a single coil the other 3 are pairs. The 484' marker coil pair are variable frequency coils used to define long, short or normal stops for trains less then 8 cars long.

This would include maximum speed commands, the destination codes, and if the train should wait for schedule adjustment.

Yes.

If anyone knows about these sort of things it will help me better understand how the system is controlled which will help me to model how it works in software.

One must understand one simple fact, the signal system WMATA uses is the same basic relay logic block signaling system that has been used on railroads for over 150 years. Coded audio frequency track circuits for the transmission of signal aspects (speed commands) has been in use for over 80 years. The wayside part of the system is proven technology. The newest part of the technology is the using of the transmitted speed commands to automate the movement of the trains. All of the hardware that performs the automatic control of trains is aboard the trains.

I hope the above was of some use.
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Re: Destination codes and train control systems

Postby drwho9437 » Mon Jul 14, 2014 7:35 pm

Thanks for the info.

One note to clarify though. Yes hardware did fail in the red line crash but you can see in Appendix B on page 152 that the software could have been written in a way that would not have allowed the crash either.

"The tag for train 214 has been dropped from the display as a result of preprogrammed Advanced Information Management system algorithms designed to eliminate false train indications."

It was a real train and it was deleted. Yes the track circuit didn't show where it was, but it had to be somewhere. The fact that the system allows for deletion of trains without any consideration of the topology of the network is dumb. Very dumb.

From an engineering point of view its not completely ridiculous because such a system design allows you to deploy the same controls on lots of different configurations without really caring much about if they are say shared between two operators that might not talk to each other. You can test such a system longer for failure because it isn't custom.

However, if WMATA knows that it adds train X to line Y and there are only junctions it can turn and A, B, C, then having an algorithm that can delete a real train rather than saying: hey wait this train disappeared everyone slow down to 10 MPH until I know where it is, is in this engineer's opinion dumb.
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Re: Destination codes and train control systems

Postby JDC » Tue Jul 15, 2014 8:02 am

drwho9437 wrote:Thanks for the info.

One note to clarify though. Yes hardware did fail in the red line crash but you can see in Appendix B on page 152 that the software could have been written in a way that would not have allowed the crash either.

"The tag for train 214 has been dropped from the display as a result of preprogrammed Advanced Information Management system algorithms designed to eliminate false train indications."

It was a real train and it was deleted. Yes the track circuit didn't show where it was, but it had to be somewhere. The fact that the system allows for deletion of trains without any consideration of the topology of the network is dumb. Very dumb.

From an engineering point of view its not completely ridiculous because such a system design allows you to deploy the same controls on lots of different configurations without really caring much about if they are say shared between two operators that might not talk to each other. You can test such a system longer for failure because it isn't custom.

However, if WMATA knows that it adds train X to line Y and there are only junctions it can turn and A, B, C, then having an algorithm that can delete a real train rather than saying: hey wait this train disappeared everyone slow down to 10 MPH until I know where it is, is in this engineer's opinion dumb.


I, too, thought it was a bit scary that Metro's system is "OK" with a train disappearing. Now, maybe if a train appeared out of nowhere (a ghost train) and disappeared a minute later, than I can see the system automatically deleting it as a ghost train. But, if a train has been logged in the system for minutes, let alone hours, and it suddenly disappears (not into a yard), than I think the system should raise bloody hell.
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Re: Destination codes and train control systems

Postby Sand Box John » Tue Jul 15, 2014 9:44 pm

"JDC"
I, too, thought it was a bit scary that Metro's system is "OK" with a train disappearing. Now, maybe if a train appeared out of nowhere (a ghost train) and disappeared a minute later, than I can see the system automatically deleting it as a ghost train. But, if a train has been logged in the system for minutes, let alone hours, and it suddenly disappears (not into a yard), than I think the system should raise bloody hell.


It did through various alarms and logs. The problem was the variety of alarms had varying importance and became background noise.

Below is a transcript of the alarm log of track circuit (B2-304) during the minutes leading up to the wreck, the track circuit the stationary train was occupying, impact occurred about 16:57:03.

2009 Jun 22 16:55:06 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:55:10 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:55:15 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:55:23 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:55:29 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:55:43 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:55:56 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:55:59 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:56:04 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:56:07 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:57:01 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:57:03 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:37:44 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:38:37 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:38:38 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
2009 Jun 22 16:38:39 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Vacant
2009 Jun 22 16:38:40 EDT B06 Status FWB 01 3 05 Track Curcuit B2-304 Occupied
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