As it turns out, I was right, but I was right for the wrong reason.
Luggage is one of those words (like
grass and
shrubbery) that refers to more than one thing but is grammatically single.
My luggage is on the train.
The grass is green.
The shrubbery is tall and thick.
We don't say
luggages or
grasses or
shrubberies. In the sentence "The grass is growing fast this spring because of all the rain" the word grass refers to numerous plants, not one.
I hope that helps.
~
R