Collective motion of caged particles is studied by calculating correlations of elongations (i.e., excess distances between two tagged particles) in a one-dimensional colloidal system, with the focus on the effect of overtaking events by which particles can hop out of the cage. It is shown analytically and verified numerically that the effect of overtaking is more prominent in shorter lengthscales, and also that the two-time elongation correlation exhibits ageing behavior due to overtaking.