"I thought you were going to that train thing tonight"
"I was, but I changed my mind"
"But that's all you've talked about for the last few weeks"
" I know, I know. I thought it would be nicer to spend Saturday night with you. Besides there was all that expense. You know the travelling, the food, some drink and the overnight stay. So I decided against it and decided on a nice Saturday night with you instead.
So as I am staying in with you tonight, would you mind if I bought some wagons for the layout with the money instead?"
(Nailed it, me thinks)