I say, and with God's success: It is permissible to pray on an airplane and a train, whether present or traveling, standing or sitting, whether it is obligatory or voluntary; because it is not possible to perform it in any other way. As for the bus, one can get off and pray outside of it. If the bus driver does not stop, he should resemble the one praying out of respect for the time, and then he can make up the prayer after getting off the bus. And God knows best.