“One who was traveling in a ship or on a raft [asda] and is unable to turn and face in the direction of Jerusalem, should focus his heart opposite the Holy of Holies.” – Mishnah Berachot 4:6, sefaria.org
Commentary includes the addition of travelling by “wagon,” as well as the aforementioned ship or raft. Needless to say, since these passages were recorded in the third century, I imagine that the inclusion of other modes of travel such as motorized vehicles like cars, and trucks, as well as trains and planes could be added to the list. The original mishnah should serve as a guideline; additionally, it is important to keep in mind, focusing the heart on the Holy of Holies, wherever one is praying.
4:7 – Regarding the musaf (additional) Shemonah Esrei prayer on Shabbos and Yom Tov, the issue is raised, whether a person praying alone is required to recite the additional prayer. There are several different rulings. 1). The additional prayer may only be recited with a congregation when there is a minyan (quorum of ten). 2). The additional prayer may be recited with or without a congregation. 3). Only to be recited if there is not local congregation reciting them; in other words, if there is a local community reciting the prayers, then there is no obligation for someone praying alone to recite the musaf, since the community musaf is said on behalf of all, including the person davening (praying) alone. Otherwise, if there is no local congregation nearby where the person is praying alone, there is an obligation to pray the musaf prayer by the individual.
The Tree of Life 