I'm not surprised...if CSX can hive off "non-core" lines to agencies while retaining trackage rights during off hours, they seem increasingly prone to doing so: They usually get a substantial one-time infusion of cash, they're relieved of at least some maintenance costs, and the impact on their operations tends to be negligible (if a freight local has to operate overnight, who cares?). And do remember that CSX was willing to sell to both Tri-Rail and SunRail.
Wick Moorman of NS actually openly stated at one point that he thought government ownership with passenger priority and lines maintained for passenger service, and freight "trackage rights" as a tenant was a better business model. Even for mainlines. (NS is already the tenant on two of its mainlines, so they may be more comfortable with this than CSX.)
It's not just the relief of maintenance costs... the Class I (NS or CSX) stops paying *property taxes* on the line. If it's owned by a government, nobody pays property taxes on the land. The trackage rights easements, for whatever reason, don't generally get assessed property taxes either (though theoretically they could have them assessed).
Furthermore, the quality of track maintenance for passenger service is way above and beyond what any of the Class I freight haulers needs for freight service, so they get "overmaintained" lines for nothing.
Basically, 90% of the maintenance costs go to the government, 100% of the property tax liabilities disappear... if they've still got enough slots to run their freight business (and the agreements *always* promise that they will have enough slots to fulfill their common carrier obligation) what's not to like?
Financially it's a very good deal for them.