Sounds and more
Another big selling point in the model train industry is the sounds that they can produce! Many of the pre war trains did not have sounds and when they did they used a lot of different gimmicks to imitate the sounds, a common one was a little mechanism that was connected to an outer rail that would imitated the sound of the car moving. Modern trains nowadays have circuit boards in them for electronic sounds that can be used with Bluetooth, but back in the day they had to use mechanical tricks to get the sounds implemented. Which is why many pre war models do not have sound
As for post war, sound design in model trains became more common. A lot of the engines had whistles that were electronically powered with a separate button on your transformer to turn it on and off, this used an electric powered whistle during those times that were also purely mechanical. Another way for sound to be implemented like the sound of the wheels being moved and sound more realistic was that some passenger and box cars would have a wheel that would touch the rails also producing a sound. There were many neat features to bring life to your model railroads back in the day.