Hello, I’ve been working for 3 hours and this happened to me, I honestly don’t know how to explain it in words, so watch the video please
If anyone knows any solution or the origin of the problem please defame
I want it to be as it was in the beginning, and I don’t know where to start
(Sorry for the grammar, I speak Spanish)
Your resulting union is way to complex. Roblox is trying to simplify it. Try having each carriage be it’s own union, and then grouping all of them together.
Besides, why are you grouping it all together in the first place? Later on, if you want to move the carriages, it’ll move as a stagnant whole. From a chain to a solid pencil.
Hope this helped. 
I want it to be a union since I am using a protractor so that it moves as a single object in change if I try to make it a model and unanchored it they all fall one by one
Here is a video of how I want the train to go
I just tried to make a union of each car separately and then join them all but it still does not work
(The train is different because 1 day has passed since I recorded the video)
As @ItsMacaw already said, your union has too much detail, and studio simplifies the union so that it uses fewer vertices.
Perhaps unioning them is not the best way to do it. Maybe grouping all the carriages and setting a part on the first carriage as the whole models primary part and lerping the model using :SetPrimaryPartCFrame() would be a better way to move the train.
I’m not a scripter, I’m more of a builder, but that’s how I would do it. 
It’s probably because your computer is slow, or roblox studio is having trouble with the certain object that you have selected. This has happened to me a few times also. Basically you need to deselect the object (If you can) and then select the object you want to. The you need to select the entire thing, not all the unions inside the building.
Your union is too complex (too many parts) roblox studio has trouble with unioning (as there are gaps between parts) and is decimating the geometry (reducing the triangles count) i guess your roblox studio even stopped responding for a moment while trying union this, as it has happened with me before.
I would suggest to make small unions like the blocks that make corners 
so there is no face clipping
and then weld everything together using F3X plugin as there is a button to just weld everything to the last selected part, instead of going to each individual part and inserting a weld in the explorer
Thank you very much for your help, my train is already running
