Train trestle

Trestle

Trestles are useful as approaches to bridges over marshes and shallows
Ancestor	Beam bridge, clapper bridge
Related	None
Descendant	Viaduct
Carries	Heavy rail
Span range	Short
Material	Timber, iron, steel, reinforced concrete, prestressed concrete
Movable	No
Design effort	low
Falsework required	No

A trestle (sometimes tressel) is a rigid frame used as a support, historically a tripod used both as stools and to support tables at banquets. A trestle bridge is a bridge composed of a number of short spans supported by such frames. Since this type of bridge is sometimes called a "trestle" for short, each supporting frame is generally referred to as a bent. Timber and iron "trestles" (trestle bridges) were extensively used in the 19th century, the former making up from 1 to 3 per cent of the total length of the average railroad. In the 21st century, steel and sometimes concrete trestles are commonly used to bridge particularly deep valleys while timber trestles remain common in certain areas.

Many timber trestles were built in the 19th and early 20th centuries with the expectation that they would be temporary. Timber trestles were used to get the railroad to its destination. Once the railroad was running, it was used to transport the material to replace trestles with more permanent works, transporting and dumping fill around some trestles and transporting stone or steel to replace others with more permanent bridges.

In the later 20th century, tools such as the earthmover made it cheaper to construct a high fill directly instead of first constructing a trestle from which to dump the fill. Timber trestles remain common in some applications, most notably for bridge approaches crossing floodways, where earth fill would dangerously obstruct floodwater.

For the purposes of discharging material below, a coal trestle carried a dead-end track, rather than a bridge.

One of the longest trestle spans created was for railroad traffic crossing the Great Salt Lake on the Lucin Cutoff in Utah. It was replaced by a fill causeway in the 1960s, and is now being salvaged for its timber.

Many wooden roller coasters are built using design details similar to trestle bridges because it is so easy to make the roller coaster very high. Since loads are well distributed through large portions of the structure it is also resilient to the stresses imposed. The structure also naturally leads to a certain redundancy (provided that economic considerations are not overly dominant). Such wooden coasters, while limited in their path (not supporting loops), possess a certain ride character (owing to structural response) that is appreciated by fans of the type.