After the war, Kleinmann between 1919 and 1920 was on special assignment in
Moscow and
St. Petersburg and then in the German legation in
Vilnius attempting to secure railroad materials left behind in
Lithuania and
Latvia. In 1920, he worked first at the railway directorate in Kattowitz (today,
Katowice) and, after its transfer to
Poland, from 1922 in the Oppeln (today,
Opole) directorate under directorate president
Julius Dorpmüller, who would later go on to become
Reichsbahn General Director and
Reich Minister of Transportation. In January 1923, Kleinmann became head of
Reichsbahn operations in
Essen, and in 1924 he was appointed a Director of the
Reichsbahn. In Essen, Kleinmann became involved with members of the
Nazi Party and became their representative in transport matters. He formally joined the Party (membership number 663,996) on 1 October 1931. Following the
Nazi seizure of power, Kleinmann became the head of the Party's leadership staff on
Reichsbahn matters in the office of Deputy
Führer Rudolf Hess, where he operated as the Party's liaison to the
Reichsbahn. He also was appointed President of the
Reichsbahn directorate in
Cologne on 1 June 1933. On 25 July, he replaced as Deputy General Director of the
Reichsbahn under newly appointed General Director Dorpmüller. He also was made chief of railway security. The
Reichsbahn leadership was expected to implement the policies of
Adolf Hitler with regard to the dismissal of
Jews and
Social Democrats, and the filling of key positions with reliable Nazis. Kleinmann undertook actions to purge the management of the
Reichsbahn of employees who were considered unreliable by the Party. On 2 October 1933, Kleinmann was named as a founding member of
Hans Frank's
Academy for German Law. On 28 October 1936, he was made president of the examination office for higher engineering administrators. On 12 February 1937, he was brought on as a new
State Secretary in the Reich and Prussian Ministry of Transportation, following Dorpmuller's appointment as the new minister.
Hermann Göring, the Prussian
Minister president, then appointed Kleinmann to the
Prussian State Council. At the same time, he was appointed to the General Council of Göring's
Four-Year Plan and served as the leader of its transport work group. On 30 January 1938, the fifth anniversary of the regime coming to power, Kleinmann was awarded the
Golden Party Badge. From 1939 to 1944 he was a member of the
Circle of Friends of the Reichsführer-SS,
Heinrich Himmler. In February 1942, the
Technische Hochschule Darmstadt awarded him an
honorary doctorate of engineering. == Paramilitary membership ==