Flat bed rail car with a 2x6 buidlable tile space

[joshinils]

A train flat bed car, which you can build anything you like on just like a space platform.
This means you can't build chests or power poles on it, but it would be possible to build combinators on it, which could be communicated with via a stop when connected to the flat bed car

The flat bed car needs to be next to a cargo wagon if you want to use items for direct insertion into the entities built on it, or directly next to a fluid wagon if you want to use liquids (i.e. for flame turrets).

This concept could be used to supersede the artillery wagon, since one could opt to put an artillery turret on a flat bed wagon.
This would mean it'd be possible to put three artillery turrets on one single wagon, which sounds OP as it is.

[Tinyboss]

That's an amazingly cool idea, but it won't happen for a boring technical reason. Only certain things in the game are able to rotate freely/smoothly, because many more sprites have to be saved in memory, one for each rotation angle. This would require every buildable entity to have all these extra sprites.

You can read the FFFs about asteroids, demolishers, and pentapods for some of the discussion about rotation and sprites, and the hoops they jump through to make it work.

[Tinyboss]

ALTHOUGH...if there was a cover that closed over the flatbed while the train is in motion, and it only opened up at stops (when the flatbed car is stopped on a horizontal or vertical segment), then it could work! It feels a little wonky for vanilla, but I would absolutely play that mod.

To make it legible in game, there could be a special kind of stop that allows the flatbed to uncover and for the stuff built on it to operate, and these would only be buildable in vertical or horizontal orientation. Though that does limit the scope a bit.