There are different elements involved here. I can speak to this because I am a subway evangelist. I will touch on a few. 1- There is the issue of obedience, if one is "assumedly" and truly called to evangelize in this manner. If someone is doing it on their own, that is another matter. 2- There is the issue of wisdom, and God is no fool. Given there is a somewhat captive audience, a subway evangelist, in my experience, would not be lead by the Spirit of God to remain long in a subway car, maybe for one short stop or two at most. The message doesn't need to be long. 3- There are at times, specific people God intends/desires for to hear the gospel message. God intervenes when and where we least expect it. 4- Jesus, Paul and other apostles spoke in synagogues besides open public places. The message and teachings were not well received by all, and many were offended. The nature of the gospel often calls the messenger into places where they are not welcome or well received. This is the prophetic aspect/role of the evangelist. Consider the Old Testament prophets,. i.e. Moses, Jeremiah. The prophet/messenger must overcome the fear of persecution/rejection/harm and deliver the message nonetheless. It is not an easy call.