That sounds like a way of deterring people from taking on any new projects - I could see the attraction in stumping up for a share of a recently withdrawn locomotive (say the Class 20 that I saw on the RHHT recently)... but you're telling me that I'd be liable to preserve it to the same standards as a historic building? That's quite an undertaking... especially as some locos are bought as "Christmas trees" (to donate parts to others in the class)...
...and would this apply to all preserved stock? Who is going to decide how many in a class need to be preserved? What if nobody wants to preserve a class like a 143? Would someone decide that we need to preserve multiple types of another class given all of the different subclasses/ liveries/ alterations?
And what about the hundreds of locomotives that were preserved prior to your legislation coming in? They are lumbered with a huge liability to maintain a hulking lump of metal (which is quite different to people purchasing a property that was already a listed building).
It's well meaning but it'd result in even fewer trains being preserved.
The lazy answer is ‘how it works for listed buildings’.
Buildings are getting listed all the time after people have bought them.
As for standards I think you are confusing a NT historic monument with a listed building. You wouldn’t be told to restore it perfectly right now, just told to prevent it deteriorating any further.
It also wouldn’t be all surviving examples, just ensuring that some survive - that they don’t rust away awaiting full restoration when a cosmetic job could be done to protect them.