When u travel on the MTR or Airport express, can u notice that not everywhere have signal lighting? Even if there is a signal lighting, there is only Red / blue signal light?? If there is a need to ask the train to travel in another speed, then how can it be done?? The full name of ATP is Automation Train Protection, it uses the signal transmitters which are installed on the track to send out electrical signals to the passing train, to give instruction to the train. The instruction will be displayed by the cab signal inside the driver's cab to let the operator knows when he should accelerate and decelerate. There is also a computer on the train to monitor the train's speed to see whether its speed is below the speed limit or not. If the train's speed exceeds the speed limit, then the train will automatically brake to ensure it will not crash into the prior train.

Early ATP system simple diagram
When the train is entering a block, it will receive an electrical signal. This signal indicates that the speed limit in this block and the target speed that the train can travel in the next block e.g. the train inside this block's speed limit is 65km/h and the next is 40km/h, the cab signal will display 40km/h. When the train receives this electrical signal, the computer will check the train's speed to ensure that the train will not exceed that limit. If the speed exceeds that limit, the train will automatically brake. There is a distance between 2 trains called overlap zone (0km/h). This is used when a train operator rush the light, there is still some distance for him to brake the train to prevent it from entering the block which already has a train in it. Because the speed has already displayed in the driverˇ¦s cab, so the signal light only needs Red/ Blue colors for supplementary. And at normal, the train only needs to follow the cab signal and the signal light.

No. of un-occupied Block in Advance	Max. Safe Speed	Target Speed
1	0km/h	0km/h
2	40km/h	0km/h
3	65km/h	40km/h
4	80km/h	65km/h
5 or more	80km/h	80km/h

ATP system's other function:

• To ensure the train follows the light signal
• To prevent the train's speed over the speed limit

The ATO system in MTR
HKMTR control system has an ATO (Automation Train Operation) system, to make the train operate in auto mode. When the train operator sees the display light of * "Train Hold" is off, then he will press the "Start" button next to the door to start the train. Then, the ATO system will follow the instructions of the ATP system to drive the train to the next station.( Since the operator no need to control the movement of the train, so the position for him is so called train operator but not train driver).If the train operator doesn't follow the instruction (e.g.The display shows a red light but the operator still presses the "Start" button),the ATP system has the ability to reject the instruction ordered by the operator.

* Inter city , the "Train Hold" display light is installed inside the driver's cab but it is installed on the platform in the Airport Express Line.

Except the auto mode, the operator can choose the manual mode. Just like the above mentioned, the speed of the train in the next block will be displayed and the track arrangement can let the operator to have enough time to response. So that the train's speed can be reduced to the allowable value before it enters the next block. The train can ignore the signal and run at the limited speed 15km/h inside the rolling stock or during emergency. When the train operates in this mode, the yellow head light of the train will turn on to warn the train operator and the depot shunters that this train is not under the control of the signal.

Moreover, when the train control system contains both ATP and ATO, then the whole system will be so called ATC (Automatic Train Control).

Distance-to-go
As mentioned above, there is an overlap zone. But this zone is really a waste of the room on the track because the railway system is so busy. To increase the frequency of the trains and the ability of trainload, so the recent ATP system uses Distance-to-go as the basic operation mode.

The main difference of the recent ATP and the early ATP system is to cancel the overlap zone. But how can it be done? That is to move the Braking point a block before the original block. When the computer on the train receives the electrical signal transmitted from the Beacons (the wireless electrical transmitter on the track) and the loops (the wires which are installed along the tracks and through the continuous transmit and receive the signals, the train and the signal system can communicate to each other), it will know the train's ** position at that moment so that it can calculate the braking curve and continuously check the trainˇ¦s speed. If the computer discovers that the trainˇ¦s speed exceeds the speed of the braking curve, it will then stop the train to ensure it will not enter the occupied block. (Since in the Distance-to-go system, the overlap zone is cancelled and so there is no room reserved for the rush light train to brake. Hence, there is a need to monitor the trainˇ¦s speed to see whether it exceeds the braking curve's limit. In this system, the target speed decelerates continuously according to this braking curve. And in here, the target speed means the maximum speed the train can drive in that block)

The Beacon used by the East Rail

In the new system, the train will continuously send out the electrical signals (In the old system, trains can only receive signals), and these signals will be received by the loops which are installed on the running tracks. The computer will analyse the information and to make the effective arrangements.

The wires shown are the Loops made by the East Rail

** Inside the signal system, there is already a "MAP" showing the route of all the railway tracks. This map and plus the data about the front block (e.g. Is the block clear?) will be sent to the train's computer through the loops. When the running train passes through the Beacon, the train's receiver will receive the Beacon's information (e.g. the reference Number of the Beacon) and then send it to the computer. The train will then know its position and the next stopping point.

This is the braking curve of Distance-to-go