Global trip id

### Describe the problem

Hi,

**Context:** In Switzerland we've introduced the Swiss Journey ID (SJYID - documentaiton only in DE/FR/IT, sorry: https://www.xn--v-info-vxa.ch/de/datenmanagement/sid4pt-swiss-id-public-transport/swiss-journey-identification-sjyid).
It can be considered a "global" (**day-long valid**) trip identifier. One of the applications of this identifier is to allow mapping travel information among different mobility-data standards.

**What we do:** We will include the SJYID in our GTFS Schedule exports and intend to do the same for GTFS realtime.

**What we realized:** Discussions with other European public transport providers have shown that we may not be the only ones who need such a construct. Particularly, to ensure compatibility with other exchange formats.

**What we suggest:** Although this concept has apparently been explicitly excluded (see https://gtfs.org/realtime/reference/#message-tripdescriptor & screenshot), we suggest to re-visit this and consider including a new field "trip_global_id" that allows to combine/aggregate multiple trips.


Thank you for your input.

### Use cases

Combining GTFS realtime data with NeTEx, HRDF, etc.

### Proposed solution

Amend GTFS Schedule's trips.txt with a field "trip_global_id"
Amend GTFS realtime's TripDescriptor to include "trip_global_id" referencing the scheduled "trip_global_id"

### Additional information

_No response_