gtfs-osm-validator (FYI)

This command line software 'gtfs-osm-validator' could be an inspiration: https://gitlab.com/stalker314314/gtfs-osm-validator

From "How does NeTEx compare with GTFS?": "Google General Transport Feed Specification (GTFS) is a widely used format for distributing timetables to third parties. The NeTEx and GTFS formats should be considered as complementary, covering different stages in the data management process: NeTEx is 'upstream', GTFS is 'downstream'."