Require from_stop_id and to_stop_id in recommended transfer point transfers (empty or 0)

[skalexch]

Summary

Require from_stop_id and to_stop_id when transfer_type=0 or is empty, since it means that it's a recommended transfer point.

Describe the Problem

from_stop_id and to_stop_id are not required when transfer_type=0 or is empty, which specifies a recommended transfer point between routes.

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Additional Information

Proposed Discussion Period

We propose a discussion period of 7 days, it will end on 8 December 2025 at 23:59:59 UTC

Proposal Update Tracker

Date	Update Description
(YYYY-MM-DD)	(Brief description of the update)

Checklist

• I have read and understood the GTFS Change process.
• I have read and understood the GTFS Guiding Principles.
• I understand the Role of Advocate and the Responsibilities attached to it.
• I have signed the Contributor License Agreement (CLA).
• I have ensured that this proposal is not covered by an existing Pull Request.