The dome car is something that's almost exclusive to the US and Canada. In all the history of railroading, only five dome cars were ever built for non-american railroads. This is one of them.
All five were built by the west german state railway DB (which then stood for Deutsche Bundesbahn, german federal railway), for use in the high-profile Rheingold and Rheinpfeil luxury express trains. When the rail market moved away from luxury express trains to general high-speed intercity connections, they were sold to different companies for use in special trains, then were used in Sweden for a while. Now four of them are back in Germany, owned by different companies and all used for special trains.
This one is wearing the classic Rheingold color scheme. This later became a standard for all german long-distance trains except the most expensive ones, which is all sorts of confusing, but it doesn't look bad. It arrived in Aachen central station with a special train from Braunschweig.