I always carry a book on the tube to ensure custody of the eyes,but never listen to my MP3 player, earphones make you unavailable, people will speak to you if you are reading. Thus I can ensure I avoid inappropriate conduct and still ensure availability to people.I remember reading once some advice for priests in an old book to the effect that you shouldn't read the breviary on the train in case people were scandalised. Presumably in those days, people knew about the priest's obligation to recite the breviary and might be shocked that he hadn't said it quietly somewhere.
Nowadays, it is probably edifying for people to see the priest reading something that is obviously a holy book. It is often presumed that it is a bible. (One of the Mission Impossible films makes this mistake.) Still, I try to arrange the day so as not to have to read the breviary when on the train. It is not always possible and yesterday I knew that if I didn't say the day hours on the train, I'd end up squashing lots of the office together later.
An African man got on at Canary Wharf and sat next to me. Eventually he pointed to the heading "Psalmus 55" and said something about the Psalms. We got into a full scale conversation about the various psalms and he told me about his prayer life. I translated some of psalm 55 and we talked about that too. He was clearly not a Catholic but I felt the best thing was just to run on about God and his goodness and mercy rather than try any direct apologetics. I think he understood that these priests who wear black and read Latin psalms share his love of the scriptures and understand what it is to meditate on the love and mercy of God. When he got off at Westminster, I prayed that God would follow up his brief introduction to the Catholic Church.