The best kind of structure for riding along rails is a pair of axles that are held parallel to each other, and are separated by a distance roughly comparable to the space between the rails. While a single rigid axle containing two conical wheels would self-steer to follow curves if the rails were uniform and free of defects, it wouldn't take much of a defect to knock an axle sufficiently far away from perpendicular to the rails that one or both wheels would cease to be supported by the rails. If there are two axles that are held parallel to each other, and the distance between them is much smaller than any turning radius, such a thing can't happen absent some truly gross defects in the rails.
One could design rail cars with two rigid axles. Such cars were and still are used in Europe. The distance between axles, however, must be small relative to the minimum turning radius the cars will have to traverse, and consequently rail cars that only have two axles will need to be short enough that the portions of the rails near the front wheels are close to parallel with those near the back wheels. Otherwise, if the rails aren't parallel to each other, there would be no way for them to be perpendicular to both axles.
Longer rail cars are constructed with a pair of bogeys that will each ride the rails in the same way as would a very short two-axle rail car. The body of the car is then connected to the center of each bogey. Although the body of the car will of course have considerable mass an inertia, it will not prevent the two bogeys from following the tracks the same way as they would if they were independent vehicles.
The fact that rail cars have four axles is not generally necessitated by the need to distribute the load, but rather by the need to ensure that each axle is held parallel to another axle which is close enough that the portions of the rails near each axle will be nearly parallel. Unless it is necessary to ensure that a very heavy load is spread out e.g. between bridge trestles, increasing the space between the axles on a bogey would make it work less well, without offering any compensating advantage.