Top 5 gtfs-rt open source projects

an open directory of mobility feeds and operators — powers both Transitland v1 and v2

Collect real-time transit data and process it into a retroactive GTFS 'schedule' which can be used for routing/analysis

Tools for working with GTFS public transit data in ArcGIS

Website that detects headway discrepancy on New York City Subway system using live countdown clocks

Real-time New York City subway service map