Factor in the cost to keep them pristine and you are back to square one. I'm am yet to see a static railway museum self sustain itself let alone expand with no outside help.
Its much cheaper to keep something pristine than to restore a derelict item to that condition. I also doubt that you would be able to recover the full cost of restoring a carriage by simply selling it. The demand for operation operational heritage carriages isn't that great, plus the potential market is rather small.
Ie: once you sell a "pristine" carriage, it may result in short term financial income, but in the long term it ultimately results in a loss if you need to obtain a similar assett in the future.
As I understand it, some of these carriages have been used recently on their services so it's not as if they are really surplus.
Selling unrestored carriages is a different matter as not much money has been spent on them and they aren't making any contribution to day to day income.
I'm sure that if the Mary Valley Heritage Railway had a better alternative they would take it.
