Like so many other things in the world of SWRO, even the terms we use are often not standardized. Take the word "train", for example.
A train, in our definition, is a group of components that together deliver fresh water at an SWRO plant. Each train consists of one or more high-pressure pumps, membrane racks, a control system and usually one or more energy recovery devices (ERDs); shutdown of one of these three components forces the entire train to fail. An SWRO plant uses the same intake system to supply one or more trains, and each train can run independently or together in a parallel series.
In fact, there are at least four different types of "trains" in use today in midsize and large SWRO plants:
Single pump/single membrane rack and ERD/control system
Single pump/multiple membrane racks and ERDs/control system
Multiple pump/single membrane rack and ERD/control system
Multiple pump/multiple membrane racks and ERDs/control system