Light rail vehicles are built in a variety of designs and dimensions. In almost all cases, they are capable of being operated in coupled trains. Modern LRVs are generally much larger and heavier than their streetcar predecessors and can have axle loads just as large as, or even larger than, so-called "heavy rail" transit vehicles.
Light rail vehicles vary in the following design characteristics:
- Unidirectional versus bi-directional
- Non-articulated versus articulated and, for the latter, the location(s) and configuration of the articulation joints
- 100% high-floor versus partial low-floor (typically 70% or less) versus 100% low-floor
- Overall size (width, length, and height)
- Truck and axle positions
- Weight and weight distribution
- Suspension characteristics
- Performance (acceleration, speed, and braking)
- Wheel diameter and wheel contour
- Wheel gauge
These characteristics must be considered in the design of both the vehicle and the track structure.
1. Unidirectional/Bi-Directional
Nearly all of the legacy streetcar systems in North America that survived up through the 1960s used
unidirectional vehicles, most often the Presidents Conference Committee (PCC) streetcar. Such
“single-end” cars had operator’s controls in the forward end, doors on the right side, and a single
trolley pole current collector at the rear. At the end of the line, cars negotiated a turning loop and
ran to the opposite terminal. Because these vehicles could negotiate curves with centerline radii as
small as 35 feet [10.7 meters], the amount of real estate needed for a turning loop was relatively
small, usually only a single urban building lot. Transit companies typically found that the expense of
buying properties and building loops was small compared to the savings associated with not having
to maintain duplicate sets of control equipment in “double-end” trolley cars.
Current designs of high-capacity light rail vehicles have much larger minimum radius limitations and the amount of real estate that is required to construct a turning loop is much greater. Accordingly, while a few European light rail lines continue to use single-end, single-sided vehicles that require turning loops, most contemporary LRVs have control cabs in both ends and doors on both sides. These cars can advantageously reverse direction anywhere that a suitable crossover track or pocket track can be provided. This arrangement is usually more economical than the turning loop in terms of real estate required and has become the norm for most modern light rail transit systems. Crossovers and pocket track arrangements can often be sited within the confines of an ordinary double-track right-of-way and do not require the supplemental property acquisition needed for turning loops.
