Getting back on thread: Am I to understand the OP has a regular Lionel Generator car, and wants to see if he can buy the tree as a separate item to use with his current gen car? That Christmas Gen car was last cataloged in 2007. 12 years ago. That, and it's a one off.
I see a few of the entire car and tree at various sites.
If you want to get creative, I see a bunch of pre-lit led 12" Christmas trees, battery operated.
I'm sure with some ingenuity it could be adapted: or just run a fake line from the tree to the Gen car and light it with the battery pack.....