Occasionally on a double-track railway one platform placed between the tracks serves both of them; this " island " arrangement, as it is termed, has the advantage that more tracks can be readily added without disturbance of existing buildings, but when it is adopted the exit from the trains is at the opposite side to that which is usual, and accidents have happened through passengers alighting at the usual side without noticing the absence of a platform.
At stations on double-track railways which have a heavy traffic four tracks are sometimes provided, the two outside ones only having platforms, so that fast trains get a clear road and can pass slow ones that are standing in the station.
The branch to East Boston (1900-1904) passes beneath the harbour bed; it is the first double-track tunnel in the United States, and the first all-cement tunnel (diameter, 23.6 ft.) in the world.
At a cost of $7,200,000, the city completed in 1917 a municipal bridge of massive steel construction, double track and double deck, across the Mississippi.
The line is double track and the rope constantly in motion, the cars being attached at intervals through its length by clips or clutches; the loaded cars move in one direction, the empties in the other.
