piggyback has 2 distinct synonyms spread across 2 of its 8 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
haul truck trailers loaded with commodities on railroad cars
haul by railroad car
Synonyms, antonyms and the broader and narrower relations for piggyback come from Princeton University’s WordNet. Full sources and licences.
support on the back and shoulders
“He piggybacked her child so she could see the show”
bring into alignment with
“an amendment to piggyback the current law”
on the back or shoulder or astraddle on the hip
“she carried her child piggyback”
the act of carrying something piggyback