Train Describer (TD) Feed

Topics

Train Describer (TD) data is available only in geographical feeds, as well as a country-wide feed. Topic name are as follows:

Topic	Description
TD_ALL_SIG_AREA	All signalling areas
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_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

Overview

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 move, or 'step' in to. Some of these may be one-way, some may be two-way.

Message types

The Train Describer feed has seven message types:

CA - Berth Step, where a description moves 'from' a berth, 'to' another berth
CB - Berth Cancel, where a description is removed from a berth
CC - Berth Interpose, where a description is interposed (placed in to) a berth
CT - Heartbeat
SF - Signalling data update
SG - Signalling data refresh
SH - Signalling data refresh finished

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.

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.

Format

Messages are sent in JSON format, as follows:

{"CA_MSG"=>{"time"=>"1349696911000", "area_id"=>"SK", "msg_type"=>"CA", "from"=>"3647", "to"=>"3649", "descr"=>"1F42"}}
{"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

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 SF, SG and SG messages are called S-Class messages, and they show the status of particular signalling elements:

Signal aspects - whether the signal is displaying its most restrictive aspect or not
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.