Had an interesting experience this week. The train to ferry you to arrivals immigration was full so that no one could get on. It was full from picking up passengers at the new mid field terminal.
I would imagine that this would happen only for a train or two, as large aircraft are not deboarding continuously from the remote terminal.
This does remind me of an interesting situation I had a couple of years ago. I attended a concert at the AsiaWorld Expo site on the airport island, which is served by the Airport Express line, one stop past the airport. At the end of the concert, there were thousands and thousands of people leaving the Expo at the same time, with essentially all of them taking the Airport Express. There was a huge but well-managed line at the station, but it filled up several successive trains to maximum capacity, with all seats and standing room taken up. Then, the next stop is the airport, ostensibly to pick up arriving passengers with their luggage to take them into town. But there was zero room on the train, and nobody getting off. The look on the faces of some of the airport passengers was priceless, especially since it was 11pm. It seemed to me that they should have or could have left one carriage of the train empty as they left the Expo, so that there would be room for airport passengers, but then again I'm not in charge of the Airport Express.