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.