BUS - from Latin omnibus, which means "for everybody." The modern English "bus" is just the remnant of the original Latin dative plural ending -ibus.

TRUCK - originally used of the fixture for guns on warships (especially those that were on a swivel); it is from Latin trochus "iron ring" and Greek trokhos "wheel." The move to a wheeled heavy vehicle must be in the sense of the trochus carrying a load (on a warship), but the precise transmission is obscure. It already existed as a vehicle term in the USA after WW1, but really picked up speed in common vernacular during World War 2.

CAB - a shortened London slang from the 18th century from the French cabriolet, which was a small horse-drawn carriage. It got its name from Latin capreolus, a small goat, because the springs of the carriage made it bounce and leap like a baby goat.

TRACTOR - this is just good Latin, from the verb trahō, trahere, trāxī, tractum "to drag, haul". The -tor prefix on Latin verb roots meant "a person who does _____." Thus a tractor is a person (and then a vehicle) who does the hauling. cf gladiator (sword guy) and many many others.