Sistema Controllo Marcia Treno
Sistema di Controllo della Marcia del Treno (SCMT) is a discontinuous train Cab signalling system used in Italy. It shares many features with the Ripetizione Segnali (RS) system, the two systems co-existing and working together. The main purpose of SCMT is to control the respect of the speed limit imposed by the signal aspect and the line condition.
SCMT is divided in two parts:
- SSB: Sottosistema di bordo ("Onboard Subsystem")
- SST: Sottosistema di terra ("Ground Subsystem")
Working principles
SCMT is based upon an array of transponders called "Punto informativo" (PI from now on) placed on the tracks near signals, reduced speed zones, and other important points along the line. The PIs form the SST. When a train passes over a PI, a set of "antennas" mounted in front of the first bogie energize it through induction. The PI then passes information about the aspect of the next signal through the aforementioned antennas to the SSB.
If the system is supported by RS, and if the signal shows a restrictive aspect (anything different from Green) it has to be acknowledged by the train driver by pressing a button, else the system just gives indication of the signal on its display.
The system then elaborates a "braking curve" (curva di frenatura), that indicates the speed the train has to respect during the approach to the signal, until the "target speed" is reached. Failure of either acknowledging signal aspect or keeping the speed under the imposed braking curve/target speed causes the SSB to command emergency braking, which lasts until the speed gets below the limit.
The SSB takes several variables into account, especially when computing the braking curve, such as:
- Percentage of brake weight (Massa frenata, it measures the overall brake efficiency)
- Maximum speed limit allowed by rolling stock
- Train length
- Steep gradient
- Signal aspect
As said before, SCMT is a discontinuous system; this means that it is not aware of next signals' aspect changes unless it passes upon the relative PIs.
E.g.: if the SSB gets informed by a PI that the approaching signal is "Yellow", it then automatically assumes that the next signal is going to be "Red" and elaborates a braking curve accordingly. But if meanwhile the signal becomes "Green", the SSB does not get aware of it, and it assumes that it is red until the train passes the next PI.
This does not apply if the Ripetizione Segnali (RS) is available (Only when the train is running on a BAcc block type line). In this case the two system work together, and since RS is a continuous system, the SSB is aware of aspect changes, while still appropriately elaborating the maximum speed the train can run.
SCMT has some benefits, in respect of the RS alone:
- is usable on any block type (Italy has different types of block systems in use, and RS is only usable on Automatic Block);
- constantly checks the maximum allowed speed (RS only on the 9 codes version);
However, it has some disadvantages:
- Frequent delays due to the increased rate of emergency brakes intervention, and for the speed limits imposed which stay even if the signals' aspect changes, as said above (especially in case of signal at danger which becomes green);
- Increased stress for the wheels and braking system because of more frequent emergency braking.
- When received on a buffer stop track, in the last 200 m the SSB imposes a speed of 5 km/h; other than the delay this may cause, on certain types of coaches below this speed doors can be opened, thus introducing safety issues.
To partially solve the first two points, the "V-V-V" (stands for Verde-Verde-Verde) has been introduced. In practice, the distant, protection and departure signals of a station may be encountered by trains showing a green aspect (if some conditions are met), even if the train is scheduled to stop there, and thus is not subjected to speed restrictions imposed by the system.
Description of the SSB
Buttons:
- SCMT: manually activates or deactivates the SCMT function (usually it activates itself automatically as soon as any PI is passed).
- MAN (manovra): sets the SSB on shunting mode.
- RSC (ripetizione segnali continua): activates the RS (ripetizione segnali) system.
- SR (supero rosso): sets the SSB on "Train Trip" mode, used to pass Red signals when needed.
- PRE (prericonoscimento): lights when the train is approaching a "Proceed Slow" signal, if the driver does not press it 12 seconds before receiving an "AC" code (which is received after passing the signal) emergency brakes are applied.
- RIC (riconoscimento): lights when the train is approaching a restrictive aspect signal, if not pressed in 3 seconds, emergency brakes are applied.
- RF (riarmo freno): used to release brakes when the SSB commands emergency brakes.
RIC and PRE keys are used only when RS is also active.
The following table shows the meaning of the signal codes icons used on the display and the target speed the train has to reach according to the braking curve/signal aspect-code.
| Code/representation on the display | Meaning | Target speed (km/h) | Notes |
|---|---|---|---|
| AC | No code, also used after the train passes a proceed slow signal when RS function is present. | 150 or the speed given by the preceding signal | |
| 75 (Empty triangle) | Signal at danger | 50, then 30 (or 10) at 200 meters from the signal. If emergency braking is commanded by the SSB, brake releasing is possible only after the train comes to a full stop. | 40 km/h when goods-type braking is active |
| 120 (RV) | Next signal is clear but shows a 30, 60 or 100 km/h restriction, depending on the aspect of the preceding signal | 65 for 30 and 60 aspects, 105 for 100; then 30, 60 or 100. | Max 60 km/h when goods-type braking is active |
| 120* (100) | Next signal has to be passed at 100 km/h speed | 100 | |
| 120** (130) | Warns the driver that the next signal is clear but shows a 130 km/h speed restriction. (Not yet used) | 130 | |
| 180 (Blank) | Approaching a warning signal | 75 - 115 * | 65 km/h when goods-type braking is active |
| 180* (150 on blank background) | Warns the driver that the next signal is clear but shows a 100 km/h speed restriction. This code is also used in case of maintenance works on the line | 150 | |
| 270 (Green) | Next 2700 m are clear (about 2 block sections) | 160 - 180 * | 120 km/h when goods-type braking is active |
| 270* (VM on green background) | Next 4050 m are clear (about 3 block sections) | 200 - 230 * | |
| 270** (SV on grean background) | Next 5400 m are clear (about 4 block sections) | 230 - 250 * | |
*Depending on brake efficiency, maximum speed allowed by line and train vehicles.
These speed/signal code values are the same used on Ripetizione segnali.