Crossties, the transverse members that support the rails and hold them in alignment, were originally untreated timbers. Although concrete ties have become more common, the majority of new ties are wood treated with creosote or some other preservative injected under pressure. The use of preservatives has increased the life of ties from 5 or 6 years to 25 or 30 years or more. Advances in track engineering have included increases in the size of ties and in the number used in a given length of track, and the establishment of rigid standards of quality.
Ballast is provided to give support, load transfer and drainage to the track and thereby keep water away from the rails and sleepers. Ballast must support the weight of the track and the considerable cyclic loading of passing trains.
Individual loads on rails can be as high as 50 tonnes (55 US or short tons) and around 80 short tons on a heavy haul freight line. Ballast is made up of stones of granite or a similar material and should be rough in shape to improve the locking of stones. In this way they will better resist movement. Ballast stones with smooth edges do not work so well. Ballast will be laid to a depth of 9 to 12 inches (up to 300 mm on a high speed track). Ballast weighs about 1,600 to 1,800 kg/cu/m.
Crossties rest in a bed of gravel ballast and support the railroad tracks placed on top. Crossties are made of wood or concrete. The steel tracks are connected to the crossties by metal plates and fasteners.
Ties are bedded in a layer of ballast, formerly consisting of various materials such as earth or cinders, but today in all main-line tracks consisting of crushed stone or slag in chips of specified size.
The angular irregular shape of the fragments ensures a porous mass for good drainage, but at the same time permits interlocking, so that weight is distributed evenly over the roadbed. The depth of ballast under the ties ranges from less than 61 cm (24 in) to 76 cm (30 in).