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I was about to list my favorites, but it would take way too long. I think if I could only have one engine, one car, and a caboose and it had to be fantasy, it would be the fire locomotive, ladder car, and red Cross work caboose. My "wish" would be an engine suited to go with the old Bugs Bunny / Marvin the Martian cars. I built one till then
The lack of a serious size engine to pull all of those WB cars has bugged me forever. I'd love to see what you came up with! (I know there was the starter set with an engine, but it's obviously not going to ever pull much more than it's own set and maybe an extra car or 2.)
I'm in for Christmas trains. I buy the Lionel boxcar every year. Back when I was new to the hobby and they first started doing a lot of cars aside from just the boxcar, I bought every one of them for a year or 2. I have since slimmed down my purchases each year since that would have gotten way out of hand. I am a little surprised to say I don't own a Polar Express set.
As suggested by my reply to Adriatic's post, I'm also in for the old Warner Bros cartoon cars, and I also happen to like the Disney ones. I never got into Harry Potter (never got involved with the books/movies).
Lots of the fundraiser cars for either club divisions or museums I am sometimes into. Being originally from Long Island, I buy most of the NLOE offerings they have offered to the general public since the Meenan Oil tank car (which I think was the first offered to the general public, but I may be mistaken there). Also got hooked on the RMLI cars. I don't worry if the cars they offer ever rolled on real rails or not. I suspect the most recent boxcar for Engine 39 restoration didn't exist in real life, and I'm OK with that .
I'm not huge on specific other merchandising, but I may be attracted to a car here or there. I had some K-Line Coca Cola boxcars originally, but I lost interest in that type of item except for the cute Polar Bear and Polar Bear/Baby seal hand cars. I like those since they are sort of a seasonal mascot. (realizing the Coke polar bear is a full time thing now, but I'm pretty sure he started in holiday themed commercials).
Sports I can appreciate on a somewhat limited scale. When MTH did Subway sets for the Yankees and Mets sometime after they played in the 2000 Subway Series, I needed to get a Yankees set. As for all the different boxcars made over the years now though, I decided there was too much being made to stay focused on that.
Oh, I also like most of the Postwar operating cars, but in modern re-incarnation. Still bummed from when they cancelled the set with the Reconnaissance Car that launched the helicopter concealed inside a few years back! (I'm pretty sure that's not a real car, but I still want a modern version of one very badly!! )
-Dave