A small suggestion. I've always thought about something like this and this is something that I would really like to see come to life for a few reasons.

Simplicity.

When it comes to making a train out of props, one thing I struggle with the most is making a good coupling system. If there were separate couplers, I can simply add them to the builds without having to make something too complex.

Granted a counter argument is that your addons come with separate bogies with couplers attached to them. But when it comes to making your own bogie out of props or using a pre-made bogie model (such as Drumdevils Propper Props or Magnum's Train Model Pack), they don't come with functional couplers. Being able to add separate couplers to them would make them functional.

Another thing is that these model packs tend to come with small indents in the models that indicate where the bogies need to be attached. When it comes to attaching your bogies to the correct spots, the couplers wouldn't be outside enough to couple to another car. Of course that does depend on the model that is being used if the indent for the bogie is too far inside. Granted you can attach the bogies farther out but it just looks a little silly and ridiculous.

I took a few screenshots to show you what I mean. I'm using Magnum's as an example.

Car model with pre-made bogies:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3418948196
It looks good but doesn't have functional couplers.

Car model with SW Train Car bogies:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3418948328
Still looks good but the bogies aren't outside and can't be used or connect to another car.

Car model with SW Train Car bogies further out:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3418948511
Now it's functional and can be connected to other cars. But it does look funny when the bogies are in that position. They make it seem out of place.

Here's another example. If I am using a model where the bogie is too far inside, I can weld the coupler farther out so it can connect to other cars. To show you what I mean, I took these screenshots.

Take the block in the caution material as the separate coupler for example. It is currently welded to the bogie. That is where I would put the coupler and then weld it to the bogie so that it can be functional without having to move the bogie to a separate position.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3418956512
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3418956777

That's the overall benefit is being able to attach separate couplers to bogies made of props or pre-made bogie models.

I have put a lot of thought into this suggestion and I have put some deep thought whether there would be any counter arguments to this, plus every time I build my own train this idea always comes to my mind, so I do hope this suggestion makes sense. I truly do believe that something like this could vastly help with building prop trains. If people want to make their own bogies or use pre-made models, separate coupler entities seem like a really good option if they want to make them functional without having to build their own mechanism.