The boxy Math Wizard smiles with its oddly wide mouth. "Aha! You are a philosopher, looking to answer difficult questions with the comforting logic of mathematics! A wise man, to understand your lowly understanding and search for solace under the tutelage of one such as myself! Of course, your question has many variables. Is the passenger being observed before, during, or after the subtraction? How many dimensions is the train travelling through? And of course, is it a true subtraction, or a substitution? Can you answer any of these variable? Without more information my equation will be vague at best, misleading at worst."