Thanks for your answers.  You're both right.  It turns out there's a good explanation in Wikipedia (I should have looked here first!):

Apparently the Muldenkippwagen were developed by the DB in the early 1950s to haul small loads of particle materials as a response to the growth of truck (lorry) transport, which was taking business away from the railways.  The Talbot wagen seem to have been designed specifically for track ballast.  

According to the Wikipedia article, the DB built 4,000 Ommi 51s between 1953 and 1970.  Each Muldenkippwagen had 5 hoppers mounted high so that the load could be dumped into a truck for final delivery by road.  The center of gravity was so high that an inattentive railway worker could tip the Muldenkippwagen over, so they were later retrofitted with clamps to hold the wagon on the tracks and prevent it from tipping over.  This type of Muldenkippwagen remained in service into the 1990s.

So now I need an open truck to meet the Muldenkippwagen when it comes into the station on my layout!

Thanks again.