Operation of these trains on the AC voltage was similar to the Lionel trains that we had in the 1950s. To change direction you would push down momentarily on the speed control lever. This sent a 20 volt pulse to energize a latching relay in the locomotive. The relay switched from the forward field winding to the reverse field winding which made the motor change the direction of rotation. I prefered to have a smoother transition in direction control and improved slow speed startup.
I replaced the locomotive relays with diodes (this made the AC motors run on DC) and build transistor DC controllers. These controllers also had a pulsed DC feature that gave the Märklin locomotives really great slow speed performance, as well as smooth full speed operation.
The locomotive lighting was also changed from 12 volt bulbs to 1.5 volt. The low voltage bulbs operated from the voltage drop across two diodes wired in parallel with the bulb. This gave me a constant brightness in the lighting through all speed ranges. Lighting in the passenger cars was also set up this way.