A single LED in an alternating current circuit will only light for half the cycle and needs to be protected by a diode wired inverse-parallel.
You could use a pair of LEDs wired inverse-parallel but each one is still only be lit for half the cycle. For your usual 50hz AC sine wave from a train set controller, that can result in a noticeable flicker. DCC is not a constant frequency alternating current, but averages out at a somewhat higher frequency so the flicker is much less noticeable. If you want proper smoothed DC then you need a bridge rectifier and smoothing capacitor.
At least, that's my understanding.