I have been, it seems, watching too many wildlife documentaries.
My first reaction was they were obviously participating in a bit of social grooming. Fingers ran slowly and rhythmically through hair. Sighs were heard that might have been content but were as likely the first twilight twitterings of sleep. They were lying across each other on two seats, his head on her breast, her head on his thigh. Stockinged toes curled from kicked-off shoes. It gradually became obvious to the rest of the carriage that it was more than casual caressing. One hears of such things. Grooming became what you might call extremely social: long, lively, loud.
Two psychology undergraduates completing a practical on a train.