Interface - The Journal of Wheel/Rail Interaction

LOW-FLOOR VEHICLES

Introducing Low-Floor Vehicles into Older Transit Systems
By Roy E. Smith • March, 2006

The introduction of new, low-floor vehicles into existing North American transit systems has provided an attractive means of meeting current operating requirements. By the same token, their introduction has created a number of vehicle/track compatibility issues—particularly when these modern cars are introduced into older systems. Paramount among these issues is that the Independently Rotating Wheels (IRWs) that are used on low-floor vehicles present a significantly higher derailment potential at lower L/V ratios than conventional, fixed-axle wheelsets.

Low-floor vehicles tend also to be multi-body cars, which are more sensitive to flaws in track and the vehicle's leveling system. Even leveling systems themselves can contribute to the track-unworthiness of low-floor cars. All of these issues must be taken into account when introducing new, low-floor vehicles on older transit track, because unless the track was designed specifically for low-floor and multi-body cars, problems can arise.

North American transit track is, for the most part, built and maintained to safe standards. But the question is, safe for what kind of vehicle? What kind of vehicles did the engineers have in mind when they designed the track and the maintenance standards? For many years, this vehicle was the PCC car (named for the Electric Railway President’s Conference Committee), which was commonly used on street track in North America. PCC cars were rugged and reliable vehicles, capable of handling track irregularities with relative ease. On the other hand, the PCC car was operated at limited speeds (of approximately 30 mph), and the ride quality was not very good.

Despite improvements to vehicle design over the years, the modern, low-floor vehicles that have been introduced on old street track that was once PCC car territory have not been able to match the performance of the PCC car, in terms of speed and safety. The Massachusetts Bay Transportation Authority (MBTA), for instance, has had to restrict the speed of its new low-floor cars to 15 mph and 20 mph in many parts of its system where conventional articulated vehicles have historically operated at 40 mph and 50 mph. The reason for this disparity is poor compatibility. Something has to change—either the track or the cars—in order to bring the two into balance. The question is: Which to change? The answer is: Both.

Independently Rotating Wheels
One of the common characteristics of low-floor vehicles is their use of Independently Rotating Wheels (IRWs). Many of the challenges of operating these cars are also related to the use of IRWs. The use of IRWs has advantages. Most significant is that the floor structure of the vehicle can rest much lower on IRWs than on conventional solid-axle wheelsets. This makes it easier for cars to meet ADA (Americans with Disabilities Act) requirements.

But there are mechanical downsides to IRWs. Conventional wheelsets generate a self-steering effect because the taper of the profile allows for one wheel to run at a larger radius than the other. This self-centering effect brings the wheelset to an equilibrium on the track. IRWs do not produce this effect. Even if the wheels have tapered profiles, with no axle to connect them, there is no steering moment. Lacking this self-steering mechanism, IRWs tend to remain straight through a curve, resulting in heavy flange contact with the rail, as well as a higher L/V ratio, which produces a greater potential for wheel climb derailment. And while a conventional wheelset might move relative to the train or truck frame, the wheels remain parallel. With IRWs, it is possible for the wheels to move out of parallel alignment, which again raises the risk of derailment.

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