Sub-Train is a self-contained train withing a train. It runs almost the entire length of Snowpiercer and is divided into three categories, one for each of the Classes.
Description
Sub-Train is a monorail track design with a single track, which means only one train car runs the length per time. It is possible that horizontal switching takes place, but we've yet to see any evidence of that. The monorail system id separated into three sectors, each of one of the Classes, which means technically up to three carts in various directions can run the length of each sector.