Not being an aviation enthusiast, I am uncertain what you mean by a 'scheme for enthusiasts'. Can you explain what you mean, please?Are there any schemes for railway enthusiasts like there is for aviation enthusiasts?
Basically you join a scheme where by you agree to report anything suspicious to police and it was a joint thing between airport authority and police where by registering details and providing passport sized photos you where issued with an ID card that showed you was part of the scheme and a genuine enthusiast so if you where near an airport and someone asked what you was doing you could explain and show the card.
The point being that not only would it prove what you was doing but it helps to build relationships between the enthusiast and authorities.
Is there any schemes like this for the railway and enthusiasts?
It was called "The Ian Allan Locospotters' Club"
Network Rail have this http://www.networkrail.co.uk/aspx/777.aspx on their site for Rail Enthusiats which is the closest I've seen to a "scheme" as such - more guidelines really.
I carry a copy of that webpage when I'm photting at a station, so that when some jobsworth tells me it's illegal to take photos I can inform him of his ignorance.