LRT itself is a broad spectrum and ranges from single unit streetcars running in mixed traffic within city streets at speeds as slow as 25 mph [40 km/h] and even lower up through multiple car trains running on a totally exclusive guideway at speeds of 60 mph [100 km/h] or faster. The streetcar lines in New Orleans are representative of the lower end of this spectrum while the Metrolink system in St. Louis is a good example of the upper end. In much of Europe, these two extremes are often called “trams” and “metros.” In Germany, the terms “strassenbahn” (“street railway”) and “stadtbahn” (“city railway”) are commonly used.
It is important to note how, along any given light rail transit line, one might reasonably include guideway and track elements that are very much like a strassenbahn while a short distance away the route’s character might radically change into that of a stadtbahn. LRT is a continuum and, within the framework of the operating requirements of a given project, the LRT track designer can incorporate appropriate elements from each of the mode’s extreme characteristics plus just about anything in between.
Light rail lines are fairly distinct from metro rail systems (often called “heavy rail”). The latter are always entirely in exclusive rights-of-way, are usually designed to handle long trains of vehicles (6 to 10 cars per train is common) and have a relatively high absolute minimum operating speed along the revenue route (usually 45 mph [72 km/h] or higher). By contrast, LRT trains can operate in shared rights-of-way, very seldom exceed three cars per train, and speeds as low as 10 mph [16 km/h] are tolerated in revenue service track. These differences usually mean that LRT can be constructed at far lower cost than metro rail transit, although the passenger throughput capacity of the latter is also much higher.
If there is any one single characteristic that defines “light rail,” it is likely the ability of the vehicle to operate in mixed traffic in the street when necessary. This draws a line between the St. Louis example above and a light metro rail operation such as SEPTA’s Norristown high speed line. The operational characteristics of each route are virtually the same, but only the St. Louis vehicle could actually operate in the street if necessary. It is a very fine distinction, and, while purists may quibble with some of the finer points of this definition.
Several rail transit projects have utilized diesel-powered light railcars (also known as “diesel mechanical units” or “DMUs”), which do not meet FRA buff strength criteria. Except for the propulsion system, many of these vehicles and the guideways they run upon closely resemble the stadtbahn end of the LRT spectrum. The second edition of the Handbook will not attempt to cover all of the nuances of the DMU mode.
Throughout this volume, the words “railroad” and “railway” will appear. By “railroad” it mean standard gauge rail operations that are part of the general system of railroad transportation. This includes freight railroads and passenger railroads (such as Amtrak and the commuter rail operations in many cities). The word “railway,” on the other hand, is intended as a broader term that includes all transportation operations that utilize a vehicle guidance system based on the use of flanged steel wheels riding upon steel rails.