The Eras or Epochs

 Era numbers are almost always refered to by Roman numerals
Era I 1835-1920 This represents the period of provincial and privately owned railroads. During this time locomotive designs and car colors and lettering varied greatly.
Era II1920-1945 The period of the Deutsche Reichsbahn Gesellschaft (DRG). Standardization replaced much of the rolling stock and locomotives with new designs or rebuilds of older equipment.
Era III*1945-1970 The post-WW2 period of the separated the German Federal Railroad in West Germany (DB) and the German State Railroad in East Germany (DR)
Era IV1970-1985 UIC numbering was used on rolling stock throughtout Europe, now that interchange was common. Paint schemes, particularly on passenger trains became more colorfull.
Era V1985 onwards Merger of DB and DR into the partially privatized German Railroad, Inc. (DB AG). Modern bright color schemes.

* This is probably the most interesting period to model for a number of reasons. First, during this period motive power was increasingly provided by newer diesel and electric locomotives while steam power continued strongly. Second, in the earlier part of this era, following the distruction of much of railroads in Europe, virtually anything was used that could be put into service. This even included American engines re-gauged as needed for local service to assist in the reconstruction. This allows for a lot less "cheating" to include interesting models in an operating model railroad. The model builder also has the opertunity to mix "new" with "old" as new structures were built to replace those destroyed or damaged, or freelance with scratchbuilts representing "temporary" facilities such as an older style passenger car elevated on a framework and used as a yard tower.

Also During Eras II, III, and later in Era IV, the standardization and reorganizations resulted in new designations of locomotives with few visible upgrades or modifications other than paint schemes. Some received internal upgrades giving them increased pulling power, but the minimal changes made modeling them easy.

Examples of this are the electric locomotives E18 Era III (Marklin #88080) and 118 Era IV (Marklin #88081) or steam locomotives class 85 Era II (Marklin #88887), Era III (Marklin #88885) or with the addition of smoke deflectors also Era III (Marklin #88886).