Timber and iron trestles (i.e. bridges) were extensively used in the 19th century, particularly for railroads.Walter Loring Webb, Railroad Construction – Theory and Practice, 6th Ed., Wiley, New York, 1917; Chapter IV – Trestles, pages 194–226. In the 21st century, steel and sometimes concrete trestles are occasionally used to bridge particularly deep valleys, while timber trestles remain common in certain areas. Timber trestles remain common in some applications, most notably for bridge approaches crossing floodways, where earth fill would dangerously obstruct floodwater.
