"How's the walk from the Antwerp station to the Ruben's museum?" About 10-15 minutes along a pedestrian shopping street that runs from the train station to cathedral. And if you come to Antwerp to see Rubens' painting, check out the soaring late Gothic cathedral. One of the most under-rated in Europe, in my opinion.
"What could I see in a two-hour walking loop from the Gent station?" Use the tram system. Most of the sites lie a long distance from the train station with fairly uninteresting areas in between. In the historic city center, you can see a castle (De Gravensteen), the Grote Markt, stadhuis, the canals, and several impressive churches. Closer to the train station, a large city park contains a modern art museum with the best acroymn in Europe-SMAK- and a more traditional art museum.
"How many stations does Brugge have?" For the old section of town that most tourists come to visit, only one.