There is no doubt in my mind that drones for photography, both commercial and amateur, will become more and more common and is fast outpacing government oversight and operator education.
So the question becomes - how does this new technology and photo medium get integrated into the formal photo trains. For the immediate future, I think every sponsor or the railroad themselves is well within their rights to prohibit the use of drones during a photo charter. Think about it - if the sponsor is leery of issuing chase passes due to potential liability, wouldn't purposely allowing a paying participant to use a drone open the door to all kinds of potential liability?
Consider the following - How would you even integrate one or more drones into a photo charter? The logistics of this are challenging. I doubt the drone owners would be happy to stay far enough away to stay out of still photographer's shots (yes they could photoshop them out assuming no one was shooting film), and that would they interfere with anyone shooting video or are they expected to dub in or filter the sound track to eliminate the drone buzz? And where do you establish a safe flight line compared to the typical photo line? Does it become the sponsor's responsibility to verify that the drone operators and drones themselves are qualified and safe?
Now we all know that there are always seems to be those few that believe that everyone should stay out of there way and forget common courtesy. Just imagine with them now flying drones! It will be a regular dogfight with drones jockeying for the best position and crashing into each other - and if a fleet of drones from freeloaders approach the guard drones will have to attack to defend the photo train ...
Clearly exaggerated - but then again some of the personalities of the attendees are also exaggerated at times. Assuming you do have qualified, experienced and courteous drone operators wishing to participate - how do you reasonably accommodate them? Can you justify offering them 1/3 or a 1/4 of the photo runs for drones? Will drone and non-drone photographers feel they have gotten their money's worth or not?
This doesn't even consider the "freeloaders" or non-formal participants. I personally don't see any issue with photographing a chartered photo train with or without a drone from public property when it is done safely from either a decent distance and not in the potential background, or during re-positioning moves when non of the paying photographers are shooting.
Some of the comments suggested that since the CATS (and many other similar operations) are publicly owned, that there should be open access. I will say that if you are safely photographing something from public lands you may have the right to do so, though it may be discourteous to freeload on a photo charter and certainly to purposely interfere with the paying participants efforts to get the opportunity to shoot the photos/video that they paid for. And I will say that if the owning railroad chooses to restrict access to their owned access roads or right of way, any freeloader purposely entering posted property could be subject to being cited for trespassing. This would be the obvious extreme measure, but a railroad (common carrier or not, public or private owned) does have the right to control access and use of its property, and to some extent is expected to do so by their insurance carrier and the Department of Homeland Security.
Now if anyone thinks that they have the right to enter any property because it is publicly owned or use a drone to photograph the same, I suggest that you try doing that during a commercial film shoot using public property or at Area 51 and let me know how well that works out.