A track circuit operating device (TCOD, or T-COD in the Rule Book) is a device fitted across the two rails that deliberately short-circuits a track circuit, making the signalling system show that section of line as occupied by a train. The signal protecting the section stays at red, which gives people working on a blocked line an extra layer of protection.
Key facts
- A TCOD does not detect trains. It imitates one, by doing what a train’s wheels and axles do to a track circuit.
- It is used as additional protection during a line blockage. The Rule Book requires at least one additional protection arrangement whenever possible, and a T-COD is one of the options.
- A T-COD may only be used where the Sectional Appendix authorises it, as long as the signalling equipment is working normally, and with the signaller’s permission.
- There are manually placed T-CODs, clamped to the rails by hand, and remotely activated T-CODs, such as the ZKL 3000 RC, which can be left in place and switched on and off from a position of safety.
What is a track circuit?
A track circuit is one of the ways the signalling system knows where trains are. A low-voltage electrical current is passed through the rails of a section of track. When a train enters the section, its steel wheels and axles connect the two rails and short-circuit the current. The signalling equipment detects this, shows the section as occupied, and keeps the signal protecting it at red.
Track circuits are designed to fail safe: if the current is lost for any reason, the section is shown as occupied.
How does a TCOD work?
A TCOD connects the two rails electrically, in the same way a train’s axle does. The track circuit then “sees” a train that isn’t there, shows the section as occupied, and the signal protecting it is held at red. Any train approaching the protected section is stopped at that signal.
Because it relies on the track circuit, a TCOD only works where the line is fitted with track circuits and the signalling is working normally. The Sectional Appendix shows where T-CODs are authorised.
How is a TCOD used in a line blockage?
A line blockage is when the signaller stops trains running on a line so that people can work on it. The person in charge of the blockage (an IWA, COSS or PC) must also arrange additional protection whenever possible. Under the Rule Book (Handbook 8):
- The T-COD must be authorised for that location in the Sectional Appendix, and the signalling equipment must be working normally.
- The work must not affect the correct operation of the track circuit.
- The person in charge gets the signaller’s permission before placing or activating the T-COD.
- They tell the signaller once the T-COD has been applied or activated.
- At the end of the work, a remotely activated T-COD must not be switched off until after the signaller has been told the line blockage is no longer needed.
The TCOD does not create the line blockage. The blockage is granted by the signaller; the TCOD is an extra safeguard in case something goes wrong.
Manual and remote TCODs
| Manual T-COD | Remotely activated T-COD | |
|---|---|---|
| How it is applied | Clamped to both rails by hand for each blockage | Installed in advance, then switched on and off remotely |
| Time on the track | Someone must walk out to place and remove it each time | No need to enter the track to apply or remove protection once installed |
| Example | Clip-on rail contact clamps and cable | ZKL 3000 RC, operated through the MTinfo 3000 app |
The ZKL 3000 RC
The ZKL 3000 RC, made by Dual Inventive, is a remotely controlled T-COD. When activated, it simulates a train in the section so the track circuit shows occupied and the protecting signal is held at red. According to the manufacturer, it:
- is switched from a smartphone, tablet or computer using the MTinfo 3000 app;
- can stay installed in the track for up to four years;
- is SIL 4 rated and holds a Network Rail Certificate of Acceptance (PA05/05043).
Because it can be installed ahead of time and switched remotely, the person in charge can take the line blockage and apply the additional protection without walking out onto the track, which saves time and removes a hazardous walk.
TCODs at RSS Infrastructure
Our Track Warning Services team, led by David Hodkinson, has invested in the ZKL 3000 RC to provide additional protection in line blockages, alongside track warning systems such as ATWS, SATWS and LOWS. Read more about how the ZKL 3000 RC reduces risk.
Sources
- RSSB Rule Book: GERT8000-TS1 – General signalling regulations
- RSSB Rule Book: GERT8000-HB8 – IWA, COSS or PC blocking a line
- Dual Inventive: ZKL 3000 RC remote-controlled T-COD
This article is a general guide. On the railway, always follow the Rule Book, the Sectional Appendix and your briefing.
