We present a study of the effect of a sudden expansion in the channel cross-section on the propagation of a monodisperse train of double-emulsion droplets. We create the train of coated droplets using a flow-focusing device with two cross-flow junctions so that the length of droplets and the separation between them vary by less than 3.3%. The sudden expansion of the channel leads to a reduction in the velocity of the droplets and decreases the separation between them. In some cases this leads to the coalescence of their thin-film coatings thus resulting in the formation of finite groups of droplets, which propagate steadily. Successively formed groups within a single experiment comprised either a constant number of droplets or different numbers of droplets, e.g. 2 or 3 droplets per group. If the droplet train exhibited more significant non-uniformity then successive groups systematically had different sizes. We used three different sizes of step for which the channel cross-sectional area was in- creased by factors of 1.7, 2 and 3. Experiments performed with a step size of 3 exhibited a single transition between ungrouped droplets and long groups which we refer to as infinite. In contrast, step sizes 1.7 and 2 exhibited a series of transitions from ungrouped droplets to groups of 2, 3, 4, 5, 6 and then infinite groups. In these cases, we found that the average group length mostly increased linearly with increasing droplet length, or decreasing spacing between them. We developed a toy model of the grouping phenomenon which predicts a phase diagram in terms of droplet spacing and length for which groups of different sizes are observed. The input to the model are the velocity ratio of the droplets upstream and downstream of the step and the critical droplet separation below which the droplets coalesce. The model shows that the range of parameters for which the groups are found decreases as the number of droplets in the group increases. The toy model captures the main features of the grouping behaviour observed experimentally with the step size of 2.