Chen writes on his website,
The piece follows some rules. Every minute, it checks for new trains launched from their end stations. The train then moves towards the end of the line, with its speed set by the schedule’s estimated trip duration. Some decisions were made for musical, aesthetic, and technical reasons, such as fading out routes over time, the gradual time acceleration, and limiting the number of concurrent trains. Also, I used the weekday schedule. Some of these limitations result in subtle variations, as different trains are chosen during each 24-hour loop.
The system has changed since 1972, and some lines no longer exist. For example, the 8 train, or the Third Ave El, was shut down in 1973. The former K train was merged into other routes. I decided to run these ghost trains between 12am-2am.
This may be the most relaxing, even pleasant way to experience the subway yet. Far more so, say, than riding it with certain people. The MTA better not figure out how to charge for this.