Zug does mean train but it comes from ziehen (to pull) as others mentioned. So I am not sure whether zügig comes from the original meaning or from 'train'. The

Zug does mean train but it comes from ziehen (to pull) as others mentioned. So I am not sure whether zügig comes from the original meaning or from 'train'. The english cognate for Zug/ziehen is tug btw (follows the common t -> z shift in german)