"In your case, it might depend on whether your roads need to interchange cars at that point."
Just because there is a switch on either end of the bridge, that doesn't mean its an interchange. For example the UP and BNSF both use the UP bridge over the Red River at the Texas-Oklahoma border but there is no "interchange" there (if you are traveling on US 75 and look east as you cross the Red River you can see the bridge). The two railroads do not exchange cars there, The both just make straight away moves from their track to the bridge and back to their track.
Just because there is a switch on either end of the bridge, that doesn't mean its an interchange. For example the UP and BNSF both use the UP bridge over the Red River at the Texas-Oklahoma border but there is no "interchange" there (if you are traveling on US 75 and look east as you cross the Red River you can see the bridge). The two railroads do not exchange cars there, The both just make straight away moves from their track to the bridge and back to their track.