There's a bridge which offers a very good possibility to watch the trains on the tracks below. The bridge ist located in Ulm, Baden-Württemberg, Germany.
The pedestrian bridge crosses the tracks and can be found nearby to the mainstation. It is absolutly save for trainspotting. You have a good view on the tracks and almost to the whole train station of Ulm.
Ulm is located on the railway line from Stuttgart to Munich, over which Intercity-Express trains operate, and part of the Magistrale for Europe (trunk line) from Paris to Budapest. European cities such as Amsterdam, Budapest, Paris and Linz can be reached without transfers. Every day, about 29,000 passengers use the station. It is used daily by about 335 trains operated by Deutsche Bahn and Agilis, 75 long-distance and 260 regional trains. The station is served by local trains that are coordinated by the Donau-Iller-Nahverkehrsverbund (Danube-Iller Local Transport Association, DING).
source: [DE] Wikipedia