I had no idea that open-faced sandwiches were a thing in Austria. (Denmark, sure.) They are part of the draw at Zum Schwarzen Kameel's lauded bar, which I didn't have time to visit. With minutes to spare at the train station, I picked up the chain version from Trzesniewski, a fine enough stand in.
Choosing based on looks alone, I ended with chopped salmon, paprika (in the Hungarian sense where paprika is the spice and the red pepper, itself), mushrooms and pickles, bolstered by cream cheese and hard boiled eggs, all on thin dark bread. More like canapes than fast food, the dainty wedges classed up the train trip back to Budapest.
Trzesniewski * Multiple Locations, Vienna, Austria