In response to Jeff's post, designation as a National Historic Landmark offers no control or authority by the federal government except if the expenditure of federal funds is involved in any project that affects or impacts the particulat landmark. If federal money was involved in a road project, as an example, that had anything to do with the C&TS, then that triggers a Section 106 review by the respective SHPO who determines if the project would adversely impact the landmark. These are important because highway types as a rule are insensitive when planning their little road improvements. From sites, buildings, towns (Silverton is a designated Landmark as an example) to railroads like the D&S, these recognize exceptional historic fabric. They can be suspended or revoked if the historic fabric is compromised to the point that the intention of the designation in the first place is substantially altered. These are reviewed by field personnel from NPS every 5 or 10 years. I've participated in a couple of these reviews and it is mainly a visual survey of the landmark, then a staff report goes back to the NPS. Also, the owner, not operator, of the railroad in question has to accept the designation. For the states involved, they may balk if they perceive any threat to their flow of federal money.