[GTFS Fares v2] Update forbidden for network_id in routes.txt if networks.txt exists too

[skalexch]

Summary

This is just a simple adjustment to make sure the reference is consistent. In #405 where networks.txt and route_networks.txt were proposed, the community decided to keep a separation between network_id in routes.txt on one side and networks.txt+route_networks.txt on the other side (to maintain backward compatibility since the features were introduced at different times).

Describe the Problem

The spec mention clearly in the dataset section that networks.txt and route_networks.txt are forbidden if network_id exists in routes.txt. However, for network_id in routes.txt itself, it only says that the field is forbidden if route_networks.txt exists (ONLY, and not networks.txt as well).

Use Cases

Proposed Solution

Add networks.txt to files that forbid the existence of network_id in routes.txt

This is a documentation change, since the change is just to maintain consistency in the reference file.

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Proposed Discussion Period

In compliance with the governance, I propose a 7-day discussion period followed by 7 days for reviewing. If there's no issue, this PR can be merged directly.

Proposal Update Tracker

Date	Update Description
(2025-09-26)	Raised PR

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.