Whichever railroad owns the interchange track. Railroad ownership and maintenance is to a specific point. One railroad maintains up to the end of their track and the other railroad maintains up to the the other side of that point. Wherever that maybe on the track.
One has to also realize that they are dealing with 39 ft rails, ballast and big things, its not being cut with a scalpel. One railroad might overlap the other railroad by 20-30-40 ft.
The line of demarcation is not anything set that is a rubber stamp for every interchange. The point is that somebody owns every foot of track. If railroad A owns just to the clearance point of their switch then that means that railroad B owns all the way from their switch to the clearance point of railroad's A's switch. If you are modeling railroad A, you might see a demarcation of ballast, if your are modeling railroad B you might not see a demarcation of ballast. It could be to the clearance point, or it could be to the edge of the right of way (which could be anywhere from 25 to 200 ft or more wide). If railroad A had to build a 2 mile lead to get to railroad B's yard, the demarcation could be 2 miles away from the switch in railroad A's main.