This detailed schedule is subject to a number of factors:
- Coherence with other works carried out by the contractor: for complex works, there may be delays in the schedule due to atmospheric conditions or the unavailability of a given material, for instance
- The staggering of works along the railway line: works carried out by other contractors may also lead to delays
- The possibility of putting the line temporarily out of service (i.e. so that no trains can use the track(s) put out of service)
If one of these parameters changes, this affects the schedule. This is why the definitive schedule for works is often not set out in advance, or occasionally up to one week before the start of the works.