German model train purveyors and (to a lesser extent) railfans roughly divide German rail history into Epochs (Epochen, auf Deutsch) in a broad brush approach to classifying German railway equipment and practice. The cut-off dates between Epochs are mostly pivotal moments in German rail history (or indeed German history in general).

The commonly accepted Eras are, with dates:

Epoch I: 1835-1920
In 1920, the German State Railways (Deutsche Reichsbahn Gesellschaft) was formed from merging the former state railways (Länderbahnen). The period prior to 1920, therefore, makes an obvious and logical era distinct from the time after. Some people define an additional era, Epoch 0, from 1835 (the date of the first German railway) until about 1880 or thereabouts, because Epoch I is even more 'broad brush' than the others in terms of time frame.
Epoch II: 1920-1945
The period of the Deutsche Reichsbahn Gesellschaft from its creation from the state railways in 1920 until its dissolution along with the German state in 1945 with the final German defeat of World War 2. Some people further subdivide this into pre-Nazi, Nazi and Wartime periods.
Epoch III: 1945-1970
The post-war railways of a divided Germany. Steam locomotives are used throughout this period in both Germanies, with increasing use of diesel and electric traction, especially in the more prosperous West. Unlike in the United States and Britain, steam traction is not precipitously swept aside; rather it is a more gradual and sensible process of replacement.
Epoch IV: 1970-1985
In approximately 1970, computer based UIC numbering of locomotives and stock is introduced across mainland Europe. From a model railway point of view, this is an important date, since UIC numbered stock is obviously out of place on a layout supposed to be depicting a time before that date, and very soon after 1970 a car carrying an old style number would be an obvious anachronism. In a more general sense, it signified the increasing international flavor of rail freight and travel in Europe. Post 1970, the railway was very different.
Epoch V: 1985-present
German reunification soon resulted in a unification of the railway systems, as well as new corporate images for them. Not too long afterwards began a loosening of the traditional state control over railway companies across Europe; privatization was all the rage. New private company trains and liveries began to take hold.

In addition, some people split off the privatization period and create an Epoch VI starting some time in the mid 1990s, but this is still a matter of debate and not followed by a majority of manufacturers or individuals.

The purpose of the Epochs is simple: to allow those with a poor knowledge of rail history to still assemble a model train collection that reasonably well fits together historically. It's also a great benefit to the railfan's parents/children/spouse trying to buy them model railroad presents.

While based on pivotal moments of German rail history, the Epochs are often used to describe periods in other European countries' history too. Many of those events had parallels in other countries not too long before or after the German dates.

Serious rail historians abhor the whole idea, of course.