Freight trains which consist of isolated cars must be made into trains and divided according to their destinations. Thus the cars must be shunted several times along their route in contrast to a unit train, which carries, for example, automobiles from the plant to a port, or coal from a mine to the power plant. This shunting is done partly at the starting and final destinations and partly (for long-distance-hauling) in classification yards.
In Europe several major classification yards in Italy have never had a hump, such as Verona Porta Nuova, Foggia or Villa San Giovanni (Fascio Bolano); other large European flat yards are for example Olten (Switzerland) or Valea lui Traian (Constanţa, Romania - this is an incompleted yard with 32 tracks which was planned to be a hump yard but has no hump). In Argentina all classification yards with the exception of Villa Maria are flat yards, though some of them have approx. 30 or more tracks.
The speed of the cars rolling down from the hump into the classification bowl must be regulated because of the different natural speed of the wagons (full or empty, heavy or light freight, number of axles), the different filling of the tracks (whether there are presently few or many cars on it) and different weather conditions (temperature, wind speed and direction). As concerns speed regulation there are two types of hump yards: without or with mechanisation by retarders. In the old non-retarder yards braking was usually done in Europe by railroaders who lay skates onto the tracks, or in the USA by riders on the cars. In the modern retarder yards this work is done by mechanized "rail brakes" called retarders. They are operated either pneumatically (e.g. in the USA, France, Belgium, Russia or China) or hydraulically (e.g. in Germany, Italy or the Netherlands).
Classification bowls consist of in average 20 to 40 tracks divided into several fans or balloons of tracks, in Europe usually with eight classification tracks following a retarder in each one, often 32 tracks altogether. In the USA also many classification bowls have more than 40 tracks up to 72 which there are often divided into six to ten tracks in each balloon loop, compared with eight in Europe.
The world's largest classification yard is a hump yard: Bailey Yard in North Platte, Nebraska, USA. Other very large US hump yards are (Kansas City-) Argentine - 2nd largest, Elkhart Young Yard, (Chicago-) Clearing, (Houston-) Englewood, Waycross Rice Yard etc. Notably, in Europe, Russia and China, all important classification yards are hump yards. Europe's largest hump yard is that of Maschen near Hamburg, Germany; it is only slightly smaller than Bailey Yard. Most hump yards are single yards with one classification bowl but some, mostly very large, hump yards have two of them, one for each direction, thus are double yards, such as Maschen, Clearing, and Bailey yards.
However, due to the transfer of freight transport from rail to road and the containerization of railfreight transport for economical reasons, hump yards are generally in decline. In Great Britain, Denmark, Norway, Japan and Australia, for example, all hump yards have already been closed.