HTFS — Historical Transit Feed Specification
A draft specification — modelled on GTFS — for representing historical public-transit networks: lines, stops and schedules with explicit time-of-validity.
GTFS describes today’s transit networks; HTFS is the equivalent draft for historical ones, where every line, stop and schedule comes with an explicit time-of-validity so a network can be reconstructed for any chosen date.
The repository is archived but the design is the basis for ongoing work on transit layers inside OHM, and the spec is the right starting point for anyone modelling historical mobility data.