How can a Muslim pray Maghrib while on the bus returning from work, knowing that the time for 'Isha prayer will begin before he reaches home?

You must strive to perform the Maghrib prayer before boarding the bus, by preparing yourself for the prayer before its time, then performing it immediately upon the entry of its time individually if you do not find a congregation. If you are unable to do so, ask the bus driver or the responsible authority to stop for you to perform the prayer. If all of that is not possible, then pray on the bus if you can, or delay the Maghrib prayer to combine it with the Isha prayer upon disembarking.