Diesel engines produce both noise and emissions, with the greatest impacts occurring during acceleration and sustained running. Newer locomotives are quieter and less polluting than earlier models built in the 1970s and 1980s. Electric-powered trains normally use multiple unit cars, which are commonly coupled into pairs with a set of driving controls at the outer ends of each pair. Sets of cars may be coupled or uncoupled to form trains of varying length. Cars draw their power from an overhead wire or a third rail.
Emissions are almost nonexistant from electric commuter trains.
Noise impacts depend on the design of equipment and the condition of the tracks. There is some concern about the localized impact of electromagnetic fields associated with high-voltage electrification, but environmental groups tend to support rail electrification because of its other, broader environmental benefits. In some cities where vertical clearances allow, double deck cars are used. Other operators prefer to run maximum-length trains at all hours, and open only as many cars to the public as needed outside of rush hours.