For the past few years I had intended to analyse the dynamical problem then write a paper with the title "On the stability of drag-along baggage..." to investigate why two-wheeled luggage can develop instabilities, which can be very annoying when rushing to catch a train, plane, etc.
But researchers in Paris have recently beaten me to it! Their conclusions are interesting and (at least initially) somewhat counterintuitive. I also found an earlier, related, paper here.
So, job done! Can strike this from the "to do" list : )