The formation of the Prussian State Railway Union in 1881 encouraged the emergence of wagon classes built to standard norms. One further European milestone was the formation of the German State Railway Wagon Association on 1 April 1909. With the participation of all the German state railways, it created a common pool of goods wagons, which by the end of 1911 had no less than 560,000 wagons. In addition, they all had standardised inscriptions and red-brown livery. In order to standardise future procurements, a total of 11 wagon classes were defined (Sheet nos. A1 to A11). These wagons of the so-called standard class ("Verbandsbauart") and subsequent developments from them (the "Austauschbauart" class with interchangeable parts) dominated goods traffic in Germany up to the Second World War and had a significant impact in many other countries which acquired these wagons either through war reparations or simply because they were left behind by the Germans after the two world wars. From 1939, wagons were developed primarily from a military point of view and were known as wartime classes ("Kriegsbauart"). After the war, in East Germany, some pre-war goods wagon classes were given a new lease of life as ‘reconstructed goods wagons’ ("Reko-Güterwagen") and continued in service for several more decades. Since the Union of Private Goods Wagon Companies ("Vereinigung der Privatgüterwagen-Interessenten (VPI)") was founded in 1921, the interests of private transport organisations in Germany (including wagon hire firms, goods wagon builders and repair firms, and owners of private sidings) has been jointly represented. The Union has around 100 members who own 50,000 goods wagons. In 2007, they transported of goods. Other countries have similar organisations. Since 1922, the agreement for the mutual use of goods wagons in international traffic (RIV) has regulated the exchange of goods wagons in Europe and the Middle East. In addition, international goods wagon fleets were created in 1953 in Western Europe with the "Europ-Verband" and in 1965 in Eastern Europe with the Common Goods Wagon Park (OPW). During the second half of the 20th century, national goods wagon classes in Europe were increasingly replaced by Union internationale des chemins de fer (UIC) standard wagons. Since 1964, all goods wagons in Germany, for example, have had to be classified using the UIC goods wagon classification system.