You are reading over their shoulder, even if it doesn't actually involve reading over their shoulder because they are sitting next to you on the train. "Over someone's shoulder" is a set phrase. It has the negative connotation of a surreptitious act.
OED includes shoulder surfing, for example "The librarian wouldn't punch in the ID and passwords if you were looking over her shoulder, trying to shoulder-surf," (my emphasis) but as Chenmunka comments on the question, it's normally used for [and the OED defines it as] "the practice of surreptitiously watching a person who is using a computer, cashpoint machine, etc., in order to obtain confidential information, such as a password or personal identification number, for fraudulent purposes." Note that even that doesn't actually have to be physically looking over someone's shoulder, merely being positioned to view the information.