I don't consider a train that runs between a city and its airport a "train," to me it's a "shuttle." Like a subway, if you will.

It would never occur to me to ask someone in Chicago or NYC for a "subway schedule." All you need to know is which platform to stand on, the trains come and go all day long every few minutes.

But perhaps there are times at Narita when there are no incoming or outgoing flights, so they don't schedule trains.