Amtrak policy (Amtrak Corporate Guidelines on Photography and Video Recording).
Note that it appears that non-commercial photography is o.k. except in restricted areas. It goes on to define a restricted area as station platforms and then says ticketed passengers are exempt provided photography occurs during the time they are preparing to board or immediately after alighting from a train. The policy is widely ignored, especially in small communities, but occasionally Amtrak police or rent-a-cops in bigger stations will show their authority.
Interesting enough, if the station platform is controlled by non-Amtrak entity (such as a commuter rail system), there generally is not a ban on private non-commercial photography. There are several recent cases going through the courts where well-known photographers (including a national magazine editor) have been harassed, cited, and even arrested for photographing trains and transit from publicly accessible areas including not only station platforms but grade crossings and even adjacent roads! It's really a stupid prohibition these days as cell phones have camera/video capability.
News to me... I run from NY to Boston a couple times a week, and the foamers are out at almost every station. Never heard of anyone getting their stones broke over it.
They have scanners, mics set up, know our names... really creepy.
Train buffs are a strange breed.