For line blocking it is determined from the start, which incoming trip has to be connected to which outgoing trip. Especially in rail services, such pre-connections are often produced due to the short time between the connected trips. The reason being, that changing the vehicle pool between arrival and departure is not possible. Desired through-connections between trips are a source for such forced connections.
A forced chaining is a relation of a vehicle journey section to a follow-up vehicle journey section on a calendar day (Chained up vehicle journey sections). It is defined as a variant of a chained up vehicle journey section and ensures that this transition is kept in the results of line blocking. Line blocking therefore has to treat the thus connected vehicle journey sections (these can be transitive whole chains) like a sole performance. Forced chainings can be different for each calendar day. They therefore connect occurrences of vehicle journey sections.
By definition a maximum of 24h to 1s. is allowed to lie between the arrival of the vehicle journey section and the departure of the successor. The calendar of the successor is therefore clearly determined by the arrival time, consequently by the calendar day of the origin vehicle journey section. Forced chaining is valid if the origin vehicle journey section operates at all on the calendar day of the forced chaining if, in the described time interval, an occurrence of the destination vehicle journey section starts after the occurrence of the origin vehicle journey section and if, in addition, the vehicle combinations of the origin vehicle journey section and the destination vehicle journey section coincide (block does not have vehicle interchange) or the respective vehicle combination sets have a non-empty intersection (block has vehicle interchange).
Forced chainings are optionally considered in line blocking. In this case, as long as none of the following conditions applies, the generated blocks meet the predefined valid forced chainings:
- The same destination vehicle journey section was defined as a destination for the same calendar day in different forced chainings. The forced chaining first found is then taken, i.e. the one with the smaller key at the origin vehicle journey section.
- The end stop point of the origin vehicle journey section does not coincide with the start stop point of the destination vehicle journey section, and the time between is not enough for an empty trip or empty trips are not allowed to be generated. The block then has a forced chaining fault.
The first case can be determined through a network checking function.
If valid forced chaining applies, demands are neither made for line blocking nor for the check line block, to comply with (potentially reduced) pre- and postpreparation times. Entering a forced chaining has priority before a possible contradicting minimum layover time. The required time for an empty trip has to be met. Downtimes at depots are not allowed between forced chainings connected by vehicle journey block items.
