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To get a bit technical, the motor is acting as ballast (electrical ballast) for the diode circuit. This prevents excessive current through the diodes.

If you connected the diode circuit directly across power from the rails, they will be destroyed - and very quickly with a lot of heat and maybe even a small "explosion."

The diodes are acting as a low budget voltage regulator.? At a very low current, each diode (that is forward biased) will drop around 0.6 volts.? As current increases, that voltage remains almost the same.? But there MUST be something for any voltage above that point to be dropped across.? That is the motor.

Remember, the goal is for constant lighting with any track voltage.? With the motor as "ballast", from the point of view of the diode circuit, excess voltage is dropped across the motor, not the diode circuit.? Of course, this "excess voltage" is what will make the motor turn which is a big plus for a model railroad engine.? But the diode circuit doesn't know that.? You could replace the motor with a resistor and the diode circuit would be just as happy.

Charles E. "Chuck" Kinzer