Definitions of piggyback:

• verb:   support on the back and shoulders

  Example: "He piggybacked her child so she could see the show"

• verb:   carry on the back or shoulders

  Example: "She piggybacked a sick classmate to school"

• verb:   haul by railroad car
• verb:   haul truck trailers loaded with commodities on railroad cars
• verb:   ride on someone's shoulders or back
• adverb:   on the back or shoulder or astraddle on the hip

  Example: "She carried her child piggyback"

• adverb:   on a railroad flatcar

  Example: "The trailer rode piggyback across the country"