Add support for train groups

[dancojocaru2000]

Some trains have multiple "groups", where the same wagons / multiple units travel together for a part of the journey, then split up and continue in different directions with different numbers (or the reverse).

Some example trains are:

• IRN 1641 and IRN 1642
• IR 1630 and IR 1584

For such trains, it would be useful to implement linked trips in the transfers.txt file. That way, routing apps such as Google Maps would show a single train instead of a transfer (see attached photos).

[BodoMinea]

Just chipping in with some extra information. I don't think it's possible to generate that with the current data.gov.ro XML information source since all the mentions of the "Grupe" tag are empty (number of found is equal to the number of found) so there's no way to infer that information.

Also, even if that information would be available, transfer_types 4 and 5 (in-seat and vehicle-to-vehicle) transfers are not currently supported in Google Maps Transit:

(https://developers.google.com/transit/gtfs/reference).

That bus-to-bus routing result with staying onboard transfer you're showing is achieved by using the same block_id on the trips (see another example from Bucharest below - that's also being presented using block_id), which wouldn't work for trains since the block logic represents contiguous / entire trips being done by the same vehicle - attempting to represent such a configuration for "train groups" would fail validation due to "overlapping stop times of different trips in the same block".

But if the information would be available, it would be a great addition to the dataset output for compliant GTFS consumers (maybe OpenTripPlanner deals with transfer_types 4 and 5?).

Maybe an issue about this should be raised in the data.gov.ro comments section.