I dont think there is a law forbidding the use of mobile phone in the trains. Japanese usually think of others first before themselves so they refrain from talking on the phone. Usually they will move to a corner and speak softly.
One think I like is that you can eat in the trains and no one will leave any garbage behind.
In cafe, fast food restaurants and food court, patrons are also expected to return their trays and all of them do so automatically. Some shops even have disposable wet towels for us to wipe the tables, hence reducing the need for cleaners.
Oh by the way, in Singapore, you can also eat in the trains. Just prepare $500.