This article discusses the dimensional characteristics of the light rail vehicle. This includes not only the static vehicle at rest, but also the additional dynamic movements the LRV can make due to both resiliency and possible failures in the vehicle suspension system. The result is a definition of the vehicle dynamic envelope (VDE). The VDE, plus additional factors, defines the track clearance envelope (TCE), which sets the minimum distances between the centerline of track and any infrastructure alongside of the track as well as the minimum distances between tracks.
Vehicle Clearance Envelopes
Clearance standards for various types of railroad cars are well documented by the use of
graphics or “plates.” For railroad equipment, one standard is the common “Plate C.” Any car
whose static dimensions fit within the limits established on Plate C can travel virtually anywhere
on the North American railroad system. Transit systems do not have similar national
standards. Therefore, transit vehicle manufacturers must develop vehicles that fit within the
clearance requirements of the system for which the car is intended. Conversely, transit system
designers should, whenever possible, configure the infrastructure so as to allow clear passage
of as broad a universe of candidate LRVs as possible. While manufacturers can, in theory,
build cars to any dimension, it is usually more economical to choose vehicles that are already
in production or have at least been engineered. Therefore, the facility designer of a new
system should establish a composite vehicle clearance envelope that accommodates vehicles
from several manufacturers to maximize competitive bidding and then design the system to
accommodate those clearances.
Vehicle Static Outline
The static outline of an LRV is based on plan and cross-sectional views showing its dimensions at
rest, including many elements as Vehicle Length, Distance between Truck Centers, Distance between End Truck and Anticlimber or Bumper, Carbody Width, Carbody End Taper, and Other Static Clearance Factors.
Vehicle Dynamic Envelope/Outline
The dynamic outline of the car is more significant to the track alignment designer than the static
outline. The vehicle dynamic envelope (VDE) of an LRV describes the maximum space that the
vehicle may occupy as it moves along the track. The dynamic outline or “clearance envelope”
includes many factors due to the normal actions of the vehicle’s suspension system, such as
carbody roll (side sway) and lateral movement between stops. The dynamic outline also includes
lateral freeplay between wheels and rail with both in their maximum wear condition as well as
abnormal conditions that may result from failure of suspension elements (e.g., deflation of an air
spring).
The development of the VDE is typically the responsibility of the vehicle designer and begins with
the cross-sectional outline of the static vehicle. The dynamic outline of the vehicle is then
developed by making allowances for carbody movements that occur when the vehicle is
operating on level tangent track. These movements represent the extremes of carbody
displacement that can occur for any combination of rotational, lateral, and vertical carbody
movements when the vehicle is operating on level tangent track.
