If there is no schedule disruption, trains will run as planned, so naturally there is no need to change the arrival and departure lines. However, this is not the case when there is some kind of transportation disruption that causes trains to be delayed or cancelled. If all trains are equally delayed, the entire schedule will simply slide backward by the amount of delay compared to the scheduled schedule, and this may not lead to a change in arrival/departure number lines. However, if the degree of delay varies from train to train, the order of arrival and departure will change. This may lead to a situation in which a different train is on the track it was supposed to be on. In such a case, there is nothing that can be done without changing the arrival/departure number line. For example, the first station of the Shinkansen. If a train arrives late, there is a possibility that a subsequent train arriving or departing on the same track will be subject to a change of track number. The time required to perform on-board maintenance and to turn around and depart is not much to spare to begin with. This will inevitably delay the departure of the turnaround train. If the following train is scheduled to arrive at the same track immediately after the turnaround, it is inevitable that the following train will be at a disadvantage. A change in the relationship between the arriving train and the first train that departs after the turnaround can also cause a change in the track number. If the connection between an arriving train and a turning train changes or if a train stops and turns back at an intermediate station due to a schedule irregularity, it may happen that “X train that was originally supposed to depart from Track 17 leaves from Track 19 only today”. In addition, the disruption of the timetable may also affect the situation where a local train is waiting for a superior train. For example, if a regular train ahead is delayed and an honor train catches up with it from behind, the train may have to stop at a different station than scheduled. In order to avoid the delay, the preceding regular train would have to enter the turnaround line at the intermediate station, so the train would have to be re-numbered there. In addition, in the process of restoring the schedule to its original state, the order of departure may be changed (reordering). Since this is also a deviation from the regular schedule, it may lead to a rerouting.
In Japan, construction work on facilities is generally carried out when trains are not running, so that it does not interfere with the operation of the trains. However, this is not the case when construction work cannot be completed in a short period of time. If construction is underway at a station and a particular track is out of service due to construction, all trains arriving and departing from that track will have to be rerouted to another track. Therefore, it is necessary to change the track number. A familiar example is when the Yamanote Line or the Keihin Tohoku Line undergoes construction somewhere between Tamachi and Tabata. If the Yamanote Line is subject to construction, it is not possible to run commercial trains, so trains on the Yamanote Line will run on the Keihin-Tohoku Line tracks for this section only. Naturally, trains on the Yamanote Line will arrive and depart from platforms on the Keihin Tohoku Line (and vice versa, of course). In addition, there have been cases in which trains have been stuck due to breakdowns or damage to the cars, which have blocked some number line, leading to a change of number line. In snowfall areas, we have experienced situations where it was not possible to change the train number for each train due to a turnout non-conversion, and all trains were put in the same track.
On rare occasions, an event is held on a platform that makes it impossible to depart from or arrive at the platform, and a train that was scheduled to depart from or arrive at the platform is shifted to a different platform. Also, when dignitaries use a train, it is not entirely impossible to say that there are situations in which the platform is “cleared” for security reasons. This could be considered a kind of event, but there could also be situations where the timetable is changed and a change in numbering is necessary because additional extra trains are set up only on certain days or at certain times of the year. In general, extra trains that are pushed in at a later time are not handled as well (?). If so, the extra train will be moved to an open track. However, if there is a reason that the extra train must arrive or depart at a particular track, the regular train may be moved to a different track.
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