  Single and double paramagnetic colloidal particles are placed above a
magnetic square pattern and are driven with an external magnetic field
precessing around a high symmetry direction of the pattern. The external
magnetic field and that of the pattern confine the colloids into lanes parallel
to a lattice vector of the pattern. The precession of the external field causes
traveling minima of the magnetic potential along the direction of the lanes. At
sufficiently high frequencies of modulation only the doublets respond to the
external field and move in direction of the traveling minima along the lanes,
while the single colloids cannot follow and remain static. We show how the
doublets can induce a coordinated motion of the single colloids building
colloidal trains made of a chain of several single colloids transported by
doublets.
