Journey planners look at two things when deciding the status of a station as an interchange point for a journey: Minimum Interchange Time and Interchange Suitability.
The data format used in these PMS-maintained fields was created for the original British Rail journey planner and has been inherited by all subsequent systems, with the values for most stations as they were originally set back in 1991-1992. The PMS Support Team holds a record of permanent and temporary changes made since 2001, but no archives exist for years prior to that. Here is an example of the relevant fields as they appear in a PMS Location record:
Minimum Interchange Time
With a few agreed exceptions, the Minimum Interchange Time in PMS is the same as the “Connectional Allowance” defined in Network Rail's Timetable Planning Rules. Where a station has no value shown in the planning rules a default of 5 minutes applies.
For most stations the interchange time applies to all train operators serving that location, but variations apply at some locations as “TOC Specific Interchanges”. Under normal circumstances these values will also be as stated in Network Rail’s Planning Rules.
Interchange Suitability
The Interchange Suitability of a station can be set to one of four levels in the timetable location data exported from PMS: 0 = Not an Interchange, 1 = Small Interchange, 2 = Medium Interchange, 3 = Large Interchange.
Where it is possible to change trains at more than one station during a journey, retail systems will tend to favour a station with a higher interchange status unless other parameters are a determining factor (minimum interchange time or fares routeing guide restriction, for example). Though as the various journey planners use software from different suppliers the algorithms running in the background may alter that; some may favour the first valid interchange point regardless of the interchange weighting whilst others may try to keep passengers on a long-distance service for as far as possible before changing to a local train.
Variations
Some minimum interchange times and interchange suitability have been adjusted over the years to take account of revisions to service patterns or station redevelopment. Temporary changes may also apply to the minimum interchange time and/or suitability for engineering work and other service alterations, such as when a non-interchange station is being used as a railhead for replacement buses.
Permanent alterations are considered on an ad hoc basis, if agreed by all the train operators serving that station. Any changes that are applied may need to be reverted if found to have unexpected consequences on other journeys.