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= Train Describer (TD) Feed =
The Train Describer feed provides low-level detail about the position of trains and their train reporting number through a network of [[TD_Berths|berths]].  Usually, but not always, a berth is associated with a signal - but there are locations (such as terminal platforms at stations) where there may be more than one berth.  From each berth, there are zero or more other berths which a train description may step in to.  A step between berths represents movement of the train from one berth to another.  Some of these steps may be one-way, some may be two-way.


== Overview ==
To access the TD data feed, you must select and subscribe to the appropriate geographical topic(s) on the ''My Feeds'' page on the Data Feeds website (see the [[About_the_feeds|about the feeds]] page.)


The Train Describer feed provides low-level detail about the position of trains and their train reporting number through a network of berths.  Usually, but not always, a berth is associated with a signal - but there are locations (such as terminal platforms at stations) where there may be more than one berth.
See also the [[List_of_Train_Describers|list of train describers]] page, where you will find information on specific TD's including user contributions & documentation provided by Network Rail such as S-Class data, maps and stepping data.


From each berth, there are zero or more other berths which a train description may move, or 'step' in to.  Some of these may be one-way, some may be two-way.
= Topics =


=== Message types ===
Train Describer messages (C-Class and S-Class) are presented on a single ActiveMQ topic named '''TD_ALL_SIG_AREA'''.  Previously, they were also available on a number of separate topics however the mappings had not been updated for many years.


The Train Describer feed has seven message types:
For historical reference, the geographical topics were:


* '''CA''' - Berth Step, where a description moves 'from' a berth, 'to' another berth
{| class="wikitable"
* '''CB''' - Berth Cancel, where a description is removed from a berth
!Former topic
* '''CC''' - Berth Interpose, where a description is interposed (placed in to) a berth
!Description
* '''CT''' - Heartbeat
|-
* '''SF''' - Signalling data update
|TD_SE_SIG_AREA
* '''SG''' - Signalling data refresh
|Scotland - East
* '''SH''' - Signalling data refresh finished
|-
|TD_SW_SIG_AREA
|Scotland - West
|-
|TD_LNE_NE_SIG_AREA
|London North Eastern - North East
|-
|TD_MC_EM_SIG_AREA
|Midlands & Continental - East Midlands
|-
|TD_LNE_GN_SIG_AREA
|London North Eastern - Great Northern
|-
|TD_LNW_LC_SIG_AREA
|London North Western - Lancashire and Cumbria
|-
|TD_LNW_C_SIG_AREA
|London North Western - Central
|-
|TD_LNW_WMC_SIG_AREA
|London North Western - West Midlands Central
|-
|TD_WCS_SIG_AREA
|West Coast - South
|-
|TD_ANG_SIG_AREA
|Anglia
|-
|TD_KENT_MCC_SIG_AREA
|Kent and Midlands & Continental
|-
|TD_SUSS_SIG_AREA
|Sussex
|-
|TD_WESS_SIG_AREA
|Wessex
|-
|TD_WTV_SIG_AREA
|Western - Thames Valley
|-
|TD_WWC_SIG_AREA
|Western - West Country
|-
|TD_WWM_SIG_AREA
|Western Wales and Marches
|-
|}


The CA, CB and CC messages include a four-character train description.  At present, only class 1, 2 and 9 trains are included in the feed.
= Message types =


'''NOTE''': The S* message types are not yet available, but have been documented in the anticipation they will be included in the feeds in due course.
The Train Describer feed has seven message types, split into two classes. See individual pages for further details.


=== Format ===
* [[C_Class_Messages | C-Class Messages]] provide information about train describer berths and descriptions
* [[S_Class_Messages | S-Class Messages]] provide signalling state information


Messages are sent in JSON format, as follows:
= Cross-midnight bug =
There is a known issue with the TD feed that can cause a very small number of messages to be given an incorrect timestamp.  The NR TD feed assumes that all TD events occur on the date they are received by the feed.  This assumption is almost always correct but can be wrong for messages received <1s before midnight.  Such messages can easily be spotted and corrected by comparing the timestamp in the message to the current time (in UTC) and adjusting by 24 hours if there is a large difference.


* {"CA_MSG"=>{"time"=>"1349696911000", "area_id"=>"SK", "msg_type"=>"CA", "from"=>"3647", "to"=>"3649", "descr"=>"1F42"}}
The impact of this is very minor, usually only 3-5 messages out of the approx. 1.5-2 million published by the feed on a typical day.  However, it may affect latency monitoring of the feed, so affected messages should be corrected or ignored in such calculations.
* {"CB_MSG"=>{"time"=>"1349696911000", "area_id"=>"G1", "msg_type"=>"CB", "from"=>"G669", "descr"=>"2J01"}}
* {"CC_MSG"=>{"time"=>"1349696911000", "area_id"=>"G1", "msg_type"=>"CC", "descr"=>"2J01", "to"=>"G669"}}
* {"CT_MSG"=>{"time"=>"1349696911000", "area_id"=>"SA", "msg_type"=>"CT", "report_time"=>"1249"}}
* {"SF_MSG"=>{"time"=>"1349696911000", "area_id"=>"WJ", "msg_type=>"SF", "address"=>"3E", "data"=>"18", "report_time"=>"073814"}}
* {"SG_MSG"=>{"time"=>"1349696911000", "area_id"=>"WJ", "msg_type=>"SG", "address"=>"30", "data"=>"900000C0", "report_time"=>"073814"}}
* {"SH_MSG"=>{"time"=>"1349696911000", "area_id"=>"WJ", "msg_type=>"SH", "address"=>"30", "data"=>"900000C0", "report_time"=>"073814"}}


== S* messages ==
= Examples and Source Code =
* [https://github.com/openraildata/td-example-python3 Python example]
* [https://github.com/openraildata/td-trust-example-elixir Elixir example]


'''NOTE''': The S* message types are not yet available, but have been documented in the anticipation they will be included in the feeds in due course.
{{Navtable-DataFeeds}}


The SF, SG and SG messages are called S-Class messages, and they show the status of particular signalling elements:
[[Category:Train Describer Data]]
 
[[Category:C Class Messages]]
* '''Signal aspects''' - whether the signal is displaying its most restrictive aspect or not
[[Category:S Class Messages]]
* '''Points''' - whether the points are set normal or reverse
* '''Route set''' - the route set from a signal to another signal or bay platform
* '''TRTS buttons''' - the status of a Train Ready to Start button, used by train dispatch staff to indicate to the signaller that platform duties are complete
* '''Level crossings''' - whether they are lowered or raised
* '''Track circuits''' - whether a particular track circuit is occupied or not
 
Not all train describers provide S-Class data, and those that do can may a subset of the available types of signalling elements.  Wembley (WY) TD does not provide any S-Class data, whereas Rugby (R1 and T2) TDs provide all types of S-Class data except for level crossings.
 
S-Class data is held in train describers as a set of 2,048 locations, grouped in sets of 8 from 0x00 to 0xFF, which can hold either a '1' or a '0' depending on the status of a particular signalling element.  Each time an element changes state, an SF message is sent with the address of the group - from 0x00 to 0xFF, and the status of all locations in that group - from 0x00 to 0xFF.  It is entirely possible to receive an SF message which changes only one byte in an address.
 
Every few hours, the state of all registers is sent in a series of SG messages with addresses incrementing in groups of 0x04, and data of eight bytes (an octet).  The end of the message set is indicated by an SH message.
 
=== Interpretation ===
 
To decode the set of addresses and bits in to signalling elements, several other sets of data are required:
 
* The signalling outputs (SOP) table for the train describer
* A diagram containing signal numbers and permissible routes, point numbers, TRTS plungers, level crossings and track circuits
 
If S-Class data is released, it is anticipated that the SOP tables will be provided here in JSON format.

Latest revision as of 12:22, 1 June 2023

The Train Describer feed provides low-level detail about the position of trains and their train reporting number through a network of berths. Usually, but not always, a berth is associated with a signal - but there are locations (such as terminal platforms at stations) where there may be more than one berth. From each berth, there are zero or more other berths which a train description may step in to. A step between berths represents movement of the train from one berth to another. Some of these steps may be one-way, some may be two-way.

To access the TD data feed, you must select and subscribe to the appropriate geographical topic(s) on the My Feeds page on the Data Feeds website (see the about the feeds page.)

See also the list of train describers page, where you will find information on specific TD's including user contributions & documentation provided by Network Rail such as S-Class data, maps and stepping data.

Topics

Train Describer messages (C-Class and S-Class) are presented on a single ActiveMQ topic named TD_ALL_SIG_AREA. Previously, they were also available on a number of separate topics however the mappings had not been updated for many years.

For historical reference, the geographical topics were:

Former topic Description
TD_SE_SIG_AREA Scotland - East
TD_SW_SIG_AREA Scotland - West
TD_LNE_NE_SIG_AREA London North Eastern - North East
TD_MC_EM_SIG_AREA Midlands & Continental - East Midlands
TD_LNE_GN_SIG_AREA London North Eastern - Great Northern
TD_LNW_LC_SIG_AREA London North Western - Lancashire and Cumbria
TD_LNW_C_SIG_AREA London North Western - Central
TD_LNW_WMC_SIG_AREA London North Western - West Midlands Central
TD_WCS_SIG_AREA West Coast - South
TD_ANG_SIG_AREA Anglia
TD_KENT_MCC_SIG_AREA Kent and Midlands & Continental
TD_SUSS_SIG_AREA Sussex
TD_WESS_SIG_AREA Wessex
TD_WTV_SIG_AREA Western - Thames Valley
TD_WWC_SIG_AREA Western - West Country
TD_WWM_SIG_AREA Western Wales and Marches

Message types

The Train Describer feed has seven message types, split into two classes. See individual pages for further details.

Cross-midnight bug

There is a known issue with the TD feed that can cause a very small number of messages to be given an incorrect timestamp. The NR TD feed assumes that all TD events occur on the date they are received by the feed. This assumption is almost always correct but can be wrong for messages received <1s before midnight. Such messages can easily be spotted and corrected by comparing the timestamp in the message to the current time (in UTC) and adjusting by 24 hours if there is a large difference.

The impact of this is very minor, usually only 3-5 messages out of the approx. 1.5-2 million published by the feed on a typical day. However, it may affect latency monitoring of the feed, so affected messages should be corrected or ignored in such calculations.

Examples and Source Code


Network Rail Open Data Feeds
Data Feeds About the Feeds • Account States • Durable Subscriptions • Example Code ( PHP / C# / Java / Ruby / Node.js) • Advanced Uses • FAQ • Release Notes
RTPPM RTPPM Feed
Train Movements Train Movements Feed • Train Activation • Train Cancellation • Train Movement • Train Reinstatement • Change of Origin • Change of Identity • Change of Location • TSPEED Field • Planned Cancellations • Cancellation Codes
TD TD Feed • C-Class Messages • S-Class Messages • Train Describers • TD Berths
VSTP VSTP Feed
TSR TSR Feed • Route Codes
SCHEDULE SCHEDULE Feed • TIPLOC Records • Schedule and Location Records • Association Records • CIF Codes • How Scheduling Works • Allowances
Reference Data Reference Data Feed • TOC Codes • CIF Codes • Delay Attribution Codes • Identifying Locations (STANOX, TIPLOC, NLC and 3-Alpha Codes) • STANOX Geographical Areas • Train Planning data (BPLAN)
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