pig-a-back pig-a-back /ˈpɪg əˈbæk/

adverb

1. On the back, shoulder or astraddle on the hip  ( pickaback , pig-a-back , piggyback )
   on the back, shoulder or astraddle on the hip
    She carried her child piggyback
   she carried her child piggyback
   
2. On a railroad flatcar  ( pickaback , pig-a-back , piggyback )
   on a railroad flatcar
    The trailer rode piggyback across the country
   the trailer rode piggyback across the country