gtfs-tripify is a library for teasing train arrival times out of GTFS-RT reatime data streams. You can use it to build beautiful train arrival time visualizations like this one:
Although it works great 95% of the time, due to quirks in how GTFS-RT data is provided, this library produces errant output that's difficult to iron out completely. The quickstart in the
gtfs-tripify documentation explains how to use this library and how to interpret the outputs.
The blog post "Parsing subway rides with gtfs-tripify" is a detailed breakdown of how it works.