The turn-back station capacity is the key link of subway line capacity, and the turn-back capacity of intermediate stations and terminals directly affects the transportation capacity and efficiency of the whole system. Turn-back in front of the station refers to the turn-back operation of the train by crossing the line in front of the station. The advantage of turning back in front of the station is that it can reduce the investment in engineering construction, shorten the distance traveled by trains and reduce the number of trips used. However, when the train turns back, it will occupy the main line of this section, which will affect the blocking of subsequent trains. There is a hostile route between the outbound train and the inbound train, and there are hidden dangers, so the requirements for driving safety are relatively high. In practice, due to the high traffic density of the subway, turning back in front of the station is generally not adopted when the engineering conditions permit. However, sometimes it is possible to turn back in front of the station due to the limitation of project implementation conditions or in order to obtain better transfer conditions. For example, Xizhimen Station of Beijing Metro 13 Line and Songjiazhuang Station of Yizhuang Line of Beijing Metro all adopt turn-back in front of the station.

The type of turn-back station in front of the station is generally determined by the passenger flow and traffic density of the station. The following are several typical forms of turn-back station in front of the station. The island station can prevent passengers from choosing the platform, and no matter which lane the train stops in, passengers entering the island platform can ride smoothly. As the width of island platform is generally above 10m, and the line spacing is at least above 13 m, the distance between the turnout area in front of the station and the platform is greatly increased compared with that of the side station, the interference time of the train in the turnout area is long, and the turn-back ability is lower than that of the side station. In order to improve the turn-back ability, it is usually necessary to reduce the width of the island platform as much as possible, or choose an appropriate number of turnouts in front of the station to improve the speed of the train entering the station.

If there is a large passenger flow at the turn-back station, passengers getting on and off the bus use the island platform, and the passenger flow lines are seriously intertwined on the platform, which is not conducive to safety management. The development of signal technology has gone through three stages, from analog signal to digital signal, and then to wireless signal. Each stage has its own specific products and specific technical conditions. According to the characteristics of the system, it can be divided into three types:

(1) ATC system based on fixed block mode. Application examples include Beijing Metro 1 Line, Line 2, Batong Line and1Line 3.

(2) ATC system based on quasi-moving block mode. The application example is Beijing Metro Line 5.

(3) Air traffic control system based on communication technology. Application examples include Beijing Metro Line 4, 10 Line, Airport Line, etc.

Because the ATC system based on fixed block mode is a step control mode, it is difficult to realize the optimal control and energy-saving control of trains, and it also limits the improvement of driving efficiency, so it has been eliminated in the high-density subway project being designed at present. In order to provide passengers with a more comfortable rail transit system, there are two commonly used signal systems:

(1) Quasi-moving block ATC system based on digital track circuit.

(2) Communication-based mobile block ATC system.

The section capacity of ATC system with moving block is greater than that of quasi-moving block, but for the turn-back in front of the station, only one-way traffic is allowed in the turnout area in front of the station, so no matter where the train in the station is, the subsequent inbound trains can't pass through the turnout (entrance signal) before the route is opened. No matter whether the signal system adopts moving block or quasi-moving block, the position of the incoming signal can be obtained by the subsequent trains. Therefore, there is no difference in the influence of moving block and quasi-moving block on the reentry ability in front of the station. There are four kinds of trains in daily operation:

(1)ATO mode. ATO system automatically drives the train according to the ground speed limit information provided by ATP, and is supervised by the driver.

(2)ATP mode. The driver drives the train manually and runs according to the speed information of ATP. Once speeding, emergency braking will be implemented to ensure safe operation.

(3) Unlimited mode. The train is driven manually, relying on the ground display signal to run at the speed allowed by the line, and the driver ensures the safety of operation. Used when ATP system fails in a large area.

(4) Speed-limited manual driving mode. This mode is used for overspeed protection in places where there is no ATP ground speed information or when the ground equipment on the main line fails. The train is driven manually, with a speed limit of 25 km/h. Once it exceeds the speed limit, the ATP on board will implement emergency braking.

Unlimited mode and speed limit manual driving mode are both unconventional modes, which are rarely used in daily operation. ATO mode and ATP mode are commonly used modes. There is no manual driving in ATO mode, and the trains are all controlled by on-board equipment, so the reaction time of people can be ignored. In ATP mode, in addition to sufficient artificial reaction time, the driver's driving labor intensity should be reduced, and the train speed should be lower than the ATP speed limit curve to avoid ATP system triggering braking. Therefore, the capacity of reservation mode is lower than that of ATO system.