Parse and validate a GTFS Schedule (static) feed

Steps

1. Download the agency's GTFS zip file (required and optional .txt CSV files such as agency.txt, stops.txt, routes.txt, trips.txt, stop_times.txt, calendar.txt)
2. Check the field-by-field reference at gtfs.org/documentation/schedule/reference/ to confirm which fields are required, conditionally required, or optional
3. Parse stops.txt and confirm stop_lat/stop_lon are present for stops, stations, and entrances (location_type 0-2)
4. Join trips.txt to routes.txt via route_id and to calendar.txt/calendar_dates.txt via service_id to resolve which days each trip operates
5. If shapes.txt is present, associate shape_id from trips.txt with the ordered points in shapes.txt to render the vehicle path
6. Run the feed through a validator before shipping to catch reference and shape errors early