Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem (vol. 2, p. 450), addresses the situation of three or more people who eat a meal together while traveling on a boat, train, car or airplane (listen to audio recording for precise citation). He writes that the group in such a case should recite Birkat Ha’mazon together with a Zimun, just as they would if they ate a meal together in a usual setting.
Interestingly enough, Halacha treats these situations differently than the case of three or more people traveling together on animals. The Shulhan Aruch (Orah Haim 167:12) rules explicitly that people traveling together on animals who eat a meal together as they travel do not make a Zimun, since Zimun requires a "Kebi’ut" – an established meal in a proper, formal setting. Eating during travel lacks this quality of "Kebi’ut," and thus does not warrant a Zimun. Seemingly, this should apply to all forms of travel, yet, as we’ve seen, Halacha requires making a Zimun in the cases described above where people eat together during travel. One may have suggested distinguishing between transportation by animal – where each individual rides separately – and traveling together under one roof, such as in a car or train. However, the Mishna Berura (Rabbi Yisrael Meir Kagan, 1839-1933) cites the Magen Abraham (Rabbi Abraham Gombiner, Poland, 1637-1683) as ruling that even people traveling together in a carriage cannot combine to form a Zimun, due to the absence of "Kebi’iut." At first glance, then, we would apply this ruling even to the situations of a plane, train, boat, bus and car, as well.
Rabbi Moshe Halevi, however, claims that these situations differ from that of a carriage because of the high walls that surround motor vehicles, which transform the vehicle into a proper setting that qualifies as "Kebi’ut." The carriages spoken of by the Mishna Berura did not have high walls around them, and thus did not provide the kind of formal setting required to achieve "Kebi’ut." When people eat together in a plane, boat, train or car, they are inside a proper structure and can accurately be said to be eating together in a formal setting. As such, they require a Zimun.
Summary: If three or more people eat a meal together during travel in a bus, car, train, boat or airplane, they should make a Zimun before Birkat Ha’mazon, just as if they ate together in an ordinary setting.
