I am not sure if you are going to take a train in Japan during the COVID-19 season but there are three points not to be infected to COVID-19.
1 - Wear the facial mask on trains.
Japan is a country which did not introduce compulsory of wearing masks on any types of transportation but more than 90% of passengers still wear masks now.
2 - Refrain from talking to others
I understand that you want to talk during your trip in Japan. Then, usually voices will be much noisier on trains. Anyway keep your volume down not to disturb others.
3 - Wash your hands after getting of the trains
You should grab the handrails or straps on trains but refrain from touching your eyes, mouth, or nose on board. Then, wash you hands when you got off the trains.