TD

Train Describer (TD) Feed

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 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.

Topics

ActiveMQ topics are provided for signalling in specific geographic areas, as well as a topic representing all TD data in the country.

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

Message types

The Train Describer feed has seven message types, split into two classes.

C Class Messages: Berth Movement

Type	Name	Details
CA	Berth Step	The CA message is a 'step' message. This moves the description from the 'from' berth, in to the 'to' berth, cancelling the description in the 'from' berth and overwriting any description in the 'to' berth.
CB	Berth Cancel	The CB message is a 'cancel' message. This cancels the description in the 'from' berth.
CC	Berth Interpose	The CC message is an 'interpose' message. This inserts the description in to the 'to' berth, overwriting any description in the 'to' berth.
CT	Heartbeat	The CT message is a 'heartbeat' message, periodically sent from a train describer.

Message 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"}}

Message Fields

Field	CA	CB	CC	CT	Details
time	•	•	•	•	Message time UNIX timestamp in milliseconds since the UNIX epoch
area_id	•	•	•	•	Alphanumeric code representing the TD area that the message originates from (see list of train describers)
msg_type	•	•	•	•	Type of message Can be CA_MSG, CB_MSG, CC_MSG or CT_MSG
from	•	•			From berth (see TD berths page)
to	•		•		To berth (see TD berths page)
descr	•	•	•		Train description Four-letter alphanumeric code representing the headcode or description of the train
report_time				•	Reporting time

S Class Messages: Signalling Data

Type	Name	Details
SF	Signalling Update	Updates the signalling data for a specified set of signalling elements
SG	Signalling Refresh	Part of a set of SG messages which update the whole set of signalling data for TD area. Terminated by an SH message.
SH	Signalling Refresh Finished	Last message in a signalling refresh

S-Class messages report the status of the following 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 support S-Class data, and those that do may only support a subset of the available groups of data. The List of Train Describers shows S-Class support.

Message Format

Messages are sent in JSON format, as follows:

• {"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"}}

Message Fields

Field	SF	SG	SH	Details
time	•	•	•	Message time UNIX timestamp in milliseconds since the UNIX epoch
area_id	•	•	•	Alphanumeric code representing the TD area that the message originates from (see list of train desrcibers)
msg_type	•	•	•	Type of message Can be SF_MSG, SG_MSG or SH_MSG
address	•	•	•	Address of the signalling data being updated. For an SF_MSG, this is the individual signalling element. For an SG_MSG or SH_MSG, this is the starting address for the four bytes supplied in the data field.
data	•	•	•	Signalling data
report_time	•	•	•	Reporting time

Interpreting S-Class data

S-Class data represents the state of individual signalling elements within a TD area as bits within a set of 2,048 bits (numbered from 0x00 to 0xFF). The number of bits needed for a TD area depends on the number of signalling elements within that area. Each time an element changes state, an SF message is sent with the value of the elements in the relevant byte.

Every few hours, the state of all bits is sent as a 'refresh' in a series of SG messages with the value of groups of four bytes. These messages are terminated with an SH message.

To decode the set of addresses and bits in to signalling elements, a mapping table containing a list of bits and the signalling elements to which they refer is needed. To reference this to signals on the track layout, a diagram containing signal numbers, routes available from signals, point numbers and track circuit detail is also needed.