The ICE1 being the longer version with locomotives ("Triebköpfe", literally "driving heads") at both ends, while the ICE2 is only half as long, with only one lo

The ICE1 being the longer version with locomotives ("Triebköpfe", literally "driving heads") at both ends, while the ICE2 is only half as long, with only one locomotive, and two of them can be coupled - the restriction you mentioned only applies to these. ...but actually the ones to first use this configuration were the French with their TGV, which was then copied by the Germans and Italians for their respective high-speed trains (https://de.wikipedia.org/wiki/TGV#Allgemein https://de.wikipedia.org/wiki/TGV#Allgemein). And then the Austrians implemented a "budget version" of a high-speed train with their Railjet (https://en.wikipedia.org/wiki/Railjet https://en.wikipedia.org/wiki/Railjet) which uses an unmodified Taurus locomotive and a control car which has the same design as the locomotive.