when a shit resembles a foot-long subway, often these types of shits span over a few days, hence why "shits" is plural.
they often result in the perpetrator spending an unusually long amount of time in the bathroom.
they can also be extremely painful on some occasions
Greg: "hey man you were in there for ages, you ok?"
Mike: " nahh I've got the Subway Shits"
Greg: "ah sounds painful"