Poor suspension and improper leveling often contribute to the already inflated derailment potential for low-floor cars. Most vehicles use either a three-point or four-point leveling system. These systems typically consist of four air springs under a rigid car body—two at each end. The height of these springs is controlled by valves; the pressure must be continually adjusted in order to keep the car level. The valves detect where the carbody is, relative to the truck frame, and regulate the air spring pressure to keep the height constant as passengers enter and exit, and as the car passes through various track types and conditions.

In a three-point leveling system, two of the air springs at one end of the car are controlled independently; the other two are controlled by the same valve. In a four-point system, each valve is controlled independently. The major difference between the two is that in a three-point leveling system, the airbag pressures at one end will always exactly match. At the other end, the independent valves control the attitude of the car relative to the ground. If the car is sitting on unlevel track, such as a superelevated curve, the single-valve end keeps the height constant while the two independent valves adjust the attitude of the carbody. Airbag pressures will be different at that end—but only sufficient to overcome any unbalanced loads.

If a train with four-point leveling rests on unlevel track, the two valves at one end attempt to keep the carbody parallel to the truck frame at their end, while the valves at the other end attempt to do the same. In theory, this doesn’t present a problem on level track, but on unlevel track where, for example, one rail is tilted relative to the other, the dual valves at either end try to twist the carbody into position. Since the carbody is a rigid tube that will not twist, the valves go on pumping up diagonally opposite air springs without actually leveling the car. Once the car moves back onto level track, it takes a long time for the pressure to equalize. In practice, it almost never does. The resulting imbalance in weight distribution, which is frequently skewed to 70/30 from one side to the other can stress the vehicle's frame. It can also overload the wheels on one side and unload those on the other.

Three-point leveling is a better choice—particularly for low-floor and multi-body cars, which generate enough stability concerns without having to take suspension unbalance issues into account.

Multi-Body Cars
The multi-body car is an important aspect of the modern transit line, and most low-floor vehicles are multi-body cars. The most common type of multi-body car is a two-carbody, three-truck, single articulated car, with the center truck under the articulation joint. Three- and four-carbody, double or triple articulated vehicles are available too, but their behavior is very similar to the single articulated car, as long as the intermediate trucks coincide with the articulation joints. The cars that behave differently are the double-articulated cars with one truck under the center carbody. In a configuration like this, the center truck typically has no swivel motion relative to the carbody; it’s just a pair of axles mounted to the carbody. As a result, controlling the pitch of the center carbody is difficult. There are two solutions to this problem: The center carbody can be fastened to the other two carbodies, or the center carbody can be secured to the axle in the direction of pitch so that the axles themselves prevent forward and backward tilt.