Well, all I can say is that I always have a hard time replacing any road city-to-city connection by a rail connection too. No matter where I build the train-station (even in the center of each city).
Eventually the train will get more and more passengers but the road-connection will still be heavily used. So I guess the train-passengers are ‘new’, not ones which would use the road connection if the train wasn’t there, or road passengers which now use the train.