Plan B:
Here's what you do. Tell her you're going to build a layout in your spare time after my household duties are done (really ham this part up). Show some photos of some smaller ones but say It's going to be something like this, but don't be too specific.
Then when you have her blessing build your sections out of sight to the size you want (have it all ready and test fit). Then when she's out, scramble to assemble it. Once it's in the house your golden! When she balks at the size, you play the approval card, and say I said similar, as in it has trains and track.