# A Framework for Diagnosing Urban Rail Train Turn-Back Faults Based on Rules and Algorithms
## 3 Urban Rail Train Turn-Back
### Operating System and Communication Modules
#### 3.2.2 Automatic Turn-Back (ATB)  
After the automatic train protection (ATP) at the head end satisfies the condition of ATB, the automatic reverse (AR) lamp is always on, and the man-machine interface (MMI) shows the icon of ATB. After the driver at the head end presses the turn-back button, the AR light flashes at the head end, and the MMI icon changes to ATB and begins to output the parking brake. Then, the MMI sends a \"turn-back status\" message to the tail end. The driver then pulls out the key at the head end, which remains in its current mode, and begins to send a message to the zone controller (ZC), stating that the ATB light is blinking. The driver then presses the ATB button on the platform or the confirmation button on the cab, which sends a message to the ZC stating that the ATB button is always on, and the command \"ATB\" to the tail end. The tail-end ATP receives the command, disconnects from the ZC and computer interlock (CI), and exits the automatic switchover to ensure the ATB from the head end. At this point, the head end picks up the turn-back relay, gives the automatic train operation (ATO) permission, and relieves the parking brake. The train is automatically driven by ATO. The head end sends the ZC a message stating that the ATB button is off. The train stops on the track, and the head end sends a request to the tail end for switching, in addition to information about the location and the permission of the gate. After receiving the data, the AR light is always on, sucks up the relay in turn-back mode, and outputs the parking brake. The tail end acknowledges the turn-back request to the head end. The tail end initiates the registration with the ZC, sends the activation status to the ZC to be \"activated\", and sends the reentry status of AR to the ZC. The ZC then sends a special control message or mobile authorization to the tail end. After receiving the activation status of the tail end, the ZC sends a message to the head end stating that the tail-end registration was successful. After the first end receives the message, it sends the activation state to the ZC requesting to become \"deactivated\" and sends the \"first end deactivated\" information to the tail end. After the tail end satisfies the centralized traffic control (CTC) system upgrade condition, the CTC operation control level is upgraded and the AR lights start flashing. The end tail sends the \"end retrace\" command to the head end, and the head end is initiated with the ZC logout. At the same time, the turn-back mode relay is dropped at the head end, the AR light is turned off, and the means is converted to standby (STB) mode. When the tail end receives the message, it stops sending the AR status to the ZC, which gives the ATO permission and eases the parking brake. Then, the ZC mobile authorizes the relevant section to open, and the train arrives at the platform, stopping in the parking window. After the rear end outputs the parking brake, the driver inserts the rear-end key and presses the turn-back button. The rear AR lamp is turned off, relieving the parking brake and dropping the turn-back mode relay. The process of ATB is complete.