Once you plant the explosive, exit the building, fight any humans or vampires that stand in your way, until you reach a watertower in the courtyard. Behind it is another building, enter it. The exit is locked and will open only after you defeat the vampire inside. Once you exit this 2nd building you should be back in the train yard that you passed through on your way in. Backtrack until you reach the train station where you will meet a certain vampire who's name I won't mention because it would be a spoiler.
Hope it helps.
