A lot of ICE train routes run alongside the Autobahn. The beautiful thing about the Autobahn it’s that it’s legal to match the speed of the train. That said, it’s bad for the environment and cruising at 140mph where it’s safe to do so drinks fuel like nothing else.

I think in reality you will find that this is rather difficult to do, especially when it's one of the high-speed lines where the ICE can go 300 km/h - quite fun to watch the cars go by really rather quickly.