CFrame bugs out the whole model?

Hi there!

Video for Reference

I am trying to make a train simulator. I know what I am doing in regards to making the train move along a track. I am also aware on how to control the doors to work, so when you press a button, a FOR loop changes the CFrame of multiple objects in specified directions.

However the doors do work (in their own isolated model), as you have seen in the video linked above. However as soon as I place a block beneath, model the part to the model so the tree looks like:

-Model
    - Part 
    - Train Model 
       - Train Door's parts

Then I weld that part to the door parts. And press the button to open the doors. The door and the part attached completely bugs out and flies away.

Please help me, I've been stuck on this for DAYS.

The OPEN DOOR script

locked = script.Parent.Parent.Locked
open = script.Parent.Parent.OpenValue
door = script.Parent.Parent.Door
left = door.left
right = door.right

function onClick()
    if locked.Value == true then
        return
    elseif locked.Value == false then
        if open.Value == false then
            open.Value = true
            for i=1, 21 do
                stuff = right:GetChildren() 
                stuffL = left:GetChildren()
                for i=1, #stuff do
                    if (stuff[i].className == "Part") or (stuff[i].className == "WedgePart") or (stuff[i].className == "UnionOperation") then
                        if (stuff[i].Name ~= "Button") then
                            stuff[i].CFrame = stuff[i].CFrame * CFrame.new(0.1,0,0)
                        end
                    end
                end
                for i=1, #stuffL do
                    if (stuffL[i].className == "Part") or (stuffL[i].className == "WedgePart") or (stuffL[i].className == "UnionOperation") then
                        if (stuffL[i].Name ~= "Button") then
                            stuffL[i].CFrame = stuffL[i].CFrame * CFrame.new(-0.1,0,0)
                        end
                    end
                end
                wait(0)
            end
        elseif open.Value == true then
            return
        end
    end
end


script.Parent.Click.MouseClick:Connect(onClick)