Commuter trains on Shabbos

Hi,

A Jew may not ride a commuter train or bus on Shabbat, even if there is no charge, and it doesn't start or stop for the sake of the Jew, and it remains within the techum. Since these vehicles are normally ridden by people who are going to work or to make purchases, riding them is prohibited as marit ayin.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:44)

Have a great day,
Mordechai