ויקח אדני יוסף אותו ויתנהו אל בית הסהר…ויהי שם בבית הסוהר (וישב לז: כ) וידברו אליו פתח הבית (מקץ מג:יט)

To daven in a spot that’s designated for a mitzvah. (See explanation below by Shabbos Vort)

When Reb Aharon of Belz zt’’l, together with his brother The Bilgoray Rav zt’’l, were in transit to escape Hungary and reach Eretz Yisroel, they made a stop in Bucharest, Romania.  Before Reb Aharon stepped down from the train at the Bucharest station, he paused for awhile at the train’s door and whispered tefillahs (prayers).  The Bilgoray Rav tried hurrying up his brother, saying that Reb Aharon is keeping back other passengers from escorting the train and that it’s also late, for they still need to travel further to their destination. 

But Reb Aharon didn’t budge from his place until he finished davening.  Reb Aharon then explained to this brother why he delayed exiting the train. “When you arrive to a place where Yidden live, you need to daven for them.  So I davened at a spot where Yidden are used to doing a mitzvah. Though there wasn’t an actual mezuzah, it’s still considered a virtual place where a mitzvah is performed.”