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see the digitrax web site appnote at http://www.digitrax.com/faqdt.htm DCC is AC data pulses, the decoder has a rectifier and pulse modulator to convert the AC data packets to controlled DC pulses for the motor. If AC is applied directly to an AC motor, it could not run in reverse unless the field winding is reversed; DC motors use permanent magnets instead so they can be made smaller. Also the drive gearing allows a lot more low speed torque to be applied to the wheels with the motor running at a higher speed, kinda like first gear in a car. Direct drive would lose all this torque. I had an American Flyer S scale as a kid, it must have been AC powered since to reverse one had to fully stop the train, then apply power again to cause a stepping relay in the tender to swap field winding polarity. Then re-apply power to run in reverse; repeating the sequence would set it to forward again. That was in the days before silicon diodes were available and the usual selenium type rectifier had way too much resistance for low voltage, so AC and a variable turns ratio transformer was the only way to go other than batteries and rheostat. But they still geared the motor down. PS Mine all swirl counterclockwise. Is that correct for the Northern hemisphere? If not that Coriolis force thing just got debunked.
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