Investigation & Findings: GTFS Route ordering route_sort_order is purely cosmetic.

[kylerchin]

This investigates the column route_sort_order in the routes.txt table.

google/transit#83
Found via this history: https://gtfs.org/schedule/process

Boston MBTA

Mattapan is a distinct route branded as part of the Red Line.
Good order: Green, Orange, Blue, Red, Mattapan, Silver
Wrong order (alphabetical): Blue, Green, Mattapan, Orange, Red, Silver
Source: see MBTA GTFS URL below

(Also, MBTA uses it to force the following ordering: Subway, then BRT, then Bus, then Ferry)

This does not impact how the map is drawn, nor any routing algorithms. This is how routes should appear ordered in a stop departure board for a single station. It can be ignored in many cases, but this is supposedly what the agency wants. Many agencies do not provide it. It could also be used with ordering for "List of all routes under ". Since Catenary's departure board groups several stops across agencies / chateaus together and orders via distance, prominence, and time, we may ignore this.

tl;dr: This is purely cosmetic.

Asked by @mysteriouspresence

[kylerchin]

I think the only place where this maybe useful is two places

1. If you wish to list all routes under a particular agency in a tabular format, not a departure board which relys on geographical proximity.
2. A simplified route list from a particular stop? I don't see why honouring the route order would be more useful in this case. Either way, it's worth thinking about for this use case.