Can microtubule motors use every available track?

Flagellar assembly and function depend on cargo traveling via motors on microtubule doublets. Bertiaux, Mallet et al. (2018. J. Cell Biol https://doi.org/10.1083/jcb.201805030) find that only a subset of available doublets are used for this transport in trypanosomes, leading to questions about how and why this is achieved.