The GTFS shapes.txt file contains the actual geographic paths taken by your transit vehicles (buses, trains, etc.). The shapes.txt file is optional, but it’s important to have it so that routing apps can display the transit data nicely in a map and show an accurate representation of where your routes are. Check out this post on the Esri blog to find out how to create one for your system:
How to make shapes.txt file for GTFS dataset with ArcGIS | ArcGIS Blog
