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

Topics

Message are available in geographic feeds, as well as an country-wide feed. The topic names 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

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, CC and CT messages are referred to as C-Class, and the SF, SG and SH messages are referred to as S-Class.

C-Class

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"}}

CA message

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 message

The CB message is a 'cancel' message. This cancels the description in the 'from' berth.

CC message

The CC message is an 'interpose' message. This inserts the description in to the 'to' berth, overwriting any description in the 'to' berth.

CT message

The CT message is a 'heartbeat' message, periodically sent from a train describer.

S-Class

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"}}

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 TDs shows S-Class support.

Interpreting S-Class data

Consider the S-Class data as a set of 2,048 bits, grouped in to bytes numbered from 0x00 to 0xFF. Each bit relates to a particular signalling element.

Each time an element changes state, an SF message is sent with the value of the elements in the entire 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, several other sets of data are required:

• A mapping table containing a list of bits and the signalling elements to which they refer
• A diagram containing signal numbers, routes available from signals, point numbers and track circuit detail

These translation tables are currently in the process of being gathered.