Reingruber started his career at the Prussian Ministry for Public Works which was renamed as the Reich Ministry of Transport in 1919. He served there until 1933. Following the
Nazi rule he refused to become a member of the
Nazi Party and left the ministry. He joined the
Technical University of Dresden (TU Dresden) where he was a professor of railway and traffic engineering from 1934 to 1945. He served as its
Prorector between 1946 and 1948. On 11 October 1949 Reingruber was appointed minister of traffic to the cabinet led by
Minister President Otto Grotewohl.{{cite journal|title=Chronology|journal=Current History|date=December 1949|volume=17|issue=100 ==Later years and death==