 At the end of World War II, Jahn returned to Germany, and was a leading figure in reconstructing the trade unions.In 1949, he was elected as the first president of the new German Railwaymen's Federation, serving until 1958.From 1956, he was president of the ITF, while from 1949 he served in the Bundestag, representing Hannover-Land II for the Social Democratic Party of Germany.