My guess is that it relates to a franchise agreement that the C&NW would have made with the City of Chicago in the 1800s to use Carroll Avenue to run its tracks and trains over. If a train does not use it within X number of years then the city would probably move to legally vacate UP off and UP would lose all leverage if it wanted to sell air rights, for example. The first C&NW downtown depot was on that line and east of the North Branch of the Chicago River.
I believe this is the same reason Chicago Terminal ran trains on both ends of its C&E trackage a number of years ago despite having no customers at either end, including storing gondolas.