Quadruple-track railway

Quadruple-track railways can handle large amounts of traffic, and so are used on very busy routes or sections.

Quadrupling may be necessary when a new commuter rail service begins to operate on an existing line.

A variation of this can be found on the quadruple track section of the Main Northern line in New South Wales between Waratah and Maitland where one pair of tracks are used exclusively for coal trains and the other pair are used for passenger trains and general freight.

A similar process, but with all intercity and commuter passenger trains on the outer tracks and thru-freight trains on the inner tracks, was done by the Pennsylvania Railroad on its New York–Washington and Philadelphia–Pittsburgh mainlines prior to the takeover of operations by Amtrak and Conrail (and later Norfolk Southern).

This is somewhat still done to this day by NS, CSX, and Conrail Shared Assets trains on Amtrak-owned trackage in the Philadelphia area.

With fast trains in centre, local stations can be on the outside, eliminating staircases for half the passengers.

Further quadrupling has recently been carried out as part of the development of the Brussels Regional Express Network.

The country never implemented a quadruple-track line throughout its history, but there are plans for sections of the North–South Commuter Railway (NSCR) to be quadruplicated.