Re express services between Cumbernauld-Glasgow... Cumbernauld has 3 railway stations, Cumbernauld, Greenfaulds on one side of the town and Croy on the other side of the M80. Both Greenfaulds and Croy benefit from large car parks (Croy has 700+ spaces as is usually full Mon-Fri). Between all these stations there are 6 trains per hour to Queen St. So the trains have the competitive advantage in terms of travelling time - some trains take just 12 mins to or from Croy, although if you don't live near to the stations or dont have a car then the bus might be better.
Plus the trains have the benefit of big recent investment - electrification on both routes and new trains, and in the case of Croy longer trains too bringing more seats and capacity. And in the peaks the M80/M8 can be problematic in terms of congestion.
I think there was a proposal to review building another station at Abronhill but I dont think it got beyond that.