An iconic location to take photographs or video of German trains has to the Cologne Hauptbanhof. This is an amazingly busy station with trains constantly passing through all day, with the added bonus of the Rhine bridge in the background in one direction and the famous Dom Cathedral in the other. And as an extra bonus unlike most other photographic locations is that there are toilets, shelter from the rain and also lots of places to eat.
Compared to all the stories of trainspotters and photographers getting hassled in the UK, Germany is the complete opposite, no one seems to bat an eyelid, and when I have been there there always seems to be a small group of assorted nationalities at the end of the platform. In fact I would even like to thank the lady who provided a tissue to wipe the rain off my lens last time I was there.
DB still uses locomotives on many of its trains, and unlike the UK where the locos always seem to be at the same end of the train, there doesn't seem to be any consistency even on different trains to the same destinations. The platform ends will provide many great shots but walking out of the station and towards the Rhine crossing will provide lots of other almost trackside locations that work really well with a wide angle lens.
If you stay within the station choice of lens can be tricky, a telephoto zoom will be perfect for trains crossing the bridge, but as trains get closer you might need to swap to a wide angle zoom especially if you want to shoot trains about to cross the bridge with the Dom in the background.
If you get bored watching trains in the station, you can always take a walk across the river to Cologne Messe, or even go exploring and find the chocolate museum (Definately worth a look!)
Personally this is one of my favourite locations and I always try to stop there for a few hours every time I am passing. Arriving by train is best, but if you do decide to come by car there are quite a few car parks around the station area itself and most cost no more than a couple of Euros for a few hours.
Even for a die hard train fan, the constant procession of locomotives and high speed trains can fill memory cards in no time, but eventually you start getting the same shots of class 101s, ICEs etc, but the sheer volume of trains gives you scope to experiment with more 'arty' shots of panned trains to show a sense of movement. If you plan to spend any time here and you shoot digital an extra battery or two and lots of empty memory cards are the best things to pack, along with some warm clothing especially in winter.
Recommended to do at least once.