The first years of the German railways until about the start of the Weimar Republic.
1920 - 1945
The time of the Weimar Republic until the end of World War II.
1945 - 1968
From the end of WWII until the introduction of computer numbers (like TOPS in the UK)
1969 - 1994
From the last steam engines to the privatisation (start of the DB AG)
1994 - 2006
From the founding of the DB AG until the indroduction of 12-digit UIC numbers on locos.
Since then, notable things are the increased number of private TOCs and trains.