Sorry for the long version of the story but want to give the full perspective.
I originally purchased the Lionel Legacy GS4 Daylight #4449 from the 2016 catalog as a BTO. When the 21" passenger cars were released I ordered the entire 7 car set. I made a post or couple comments on a thread about them as I had some issues like many others. Long story short I returned them all for a full refund in late February 2018.
Since then I've kept my eyes out for a matching passenger set for my Daylight but wasn't going to jump the gun on anything. After much searching on this very site and google,I found out about the 18" aluminum cars that were made and decided that was the set I wanted. I was actually happier that there was such a nice set made shorter than 21" but not semi scale like the 15" cars. I lucked out and found the 4 car set and the 2 car add on being sold by the same seller. They were received today and I will inspect and run them this evening.
Now while searching for the passenger cars I learned about the matching 18" aluminum StationSounds diner. After TONS of searching everywhere and the most obscure pages I became aware this won't be an easy piece to find, at least not for sale.
I decided until I find one that I will repurchase a current 21" Legacy StationSounds diner and sell it if I ever acquire the 18" that matches my set.
So I had another delivery arrive today. It was supposed to be the 21" 6-83110 Legacy StationSounds diner and was advertised as such as well as the item description. It was sold without the box and I think my first piece ever without one. Upon inspection I realized the road number didn't match up to the Lionel part number and looking at the undercarriage while programming, it wasn't legacy. It also felt like it had some heft to it. Pull out the measuring tape and google the car and road number and OMG IT'S THE 18" ALUMINUM STATIONSOUNDS DINER THAT COMPLETES THE SET!!! Talk about luck. I' still sitting here in amazement.