On a model train or a toy train a truck (bogie) that is not fixed contacts the car body only at the kingpin. On a "real" train, going around a curve, the kingpin can't hold the car perfectly upright and the outside of the truck frame (specifically the outer end of the truck bolster) leans on the underside of the car body (specifically on a fixed bolster under the car). The truck must turn freely at all times, which is nearly true on a model train with just the kingpin attachment. But for the real train, a sliding bearing system is needed on each side of the kingpin to allow for the turning of the truck going around a curve. If the truck cannot turn freely, it of course derails. One system used in the past has a round bolster (actually think: pizza crust sections instead of a full circle) mounted horizontally under the car body and above the trucks. The truck bolster has little wheels at the outer ends to roll back and forth under the theoretically smooth and flat rim of this special car bolster.
At any rate, the problem mentioned for the new Red Line and Orange Line cars is that the trucks are not rotating freely enough.
The quality of the ride is more dependent on the travel path as opposed to the kind of vehicle or the kind of energy propelling it.