Who is on the train with Shimamura when he leaves the resort town for the second time, and how do they disappoint him?

Shimamura is on the train with only two other people - a middle-aged man and a young woman. He observes that when the man speaks, the woman leans forward to listen carefully. The man soon disembarks the train and the woman remains. Shimamura is disappointed that the the two were not a couple as they appeared to be.