Accurately determining the probability of various route choices is critical in understanding the actual spatiotemporal flow of commuters and the instantaneous capacity of trains and stations. Here, we report a novel procedure, based solely on the recorded tap-in tap-out ticketing data, that dictates the route choice of commuters in a rail transit system (RTS). We show that there exists a signature travel time distribution, in the form of Gumbel type 1 function, from a given origin O to a destination D. Any particular route can then be considered as a superposition of this mapping function and one can compute the probability that a specific path, over other possible paths, is taken by a commuter from O to D. The procedure is demonstrated by considering different scenarios using travel data from smart fare cards from Singapore’s RTS; results show that the forecasted characteristic profile deviates by less than 10−5 from the actual distribution. We note that our method utilizes only two parameters that can be experimentally accounted for.