Yep, definitely. Mostly with the plank wagons, but some of the vans too, I see different versions that are labeled with the suffix "with JAG" or "detailed." The JAG versions seem to have the less detailed chassis sets. Otherwise they're identical to the detailed versions.
It depends what you're after. JAG is just the scripting used for coupling and lights. It's much more robust than bluestar etc which came before it. ACS followed and JAG uses ACS as it's base. It also does a few other things if you look at the guards vans you'll see the guard move to different places as the train travels.
Basically the tops are much the same, same textures etc but the undercarriages differ. There has always been a trade off between performance and realism.
Jack Barber created a number of UK wagons that were quite high poly but he tempered that by using the same undercarriage and just putting a different top on it. Repetition in Trainz is cheap.
Jack created a number of undercarriages for me of three different lengths. It is possible to change the axle boxes, brakes, and buffers by changing the config.txt file. So if it says early detailed in the config.txt file then it uses Jack's undercarriage. These are not meant for a laptop or low powered machine.
Edward Heaps created a wooden high detailed underframe for me, these are labelled early detailed wood frame. Again you can change the axle boxes etc. I seem to recall he also created a different undercarriage. These are available in different lengths.
Then you get to the 17_5 which uses a very simple low poly undercarriage and repeats it. That way Middleton for laptops can run on a laptop. The tops are the same as the high detailed models except they all are 17.5 feet long so if you don't look too closely at the under carriage then these have the lowest machine impact.
The wagons often use the same texture layout so if there is one in one variation that you'd like to see in another you can often just clone it and copy over the texture file.
Reskins are just what people thought to create at the time. I haven't kept track of them but there are quite a few. If you'd like to try your hand there are blank versions of the wagons and vans for that matter floating around to make it fairly easy to reskin.
Think about what time period you're modelling, that will decide which underframe you want to use. TCWW isn't terribly structured so people just create what they're interested in. Look at the library for the underframe that should give you an idea of which axle boxes etc are available.
With a high end machine I'd go detailed or early detailed. With a laptop go 17_5 variations.
Have fun.
Cheerio John