Position is not a valid member of Model?

Hello, the issue is on line 27 and despite it being a part it keeps returning the error "Position is not a valid member of Model".

I'm trying to make a auto-generating track.

Problem Code:

    newMiddle.Position = Vector3.new(mainMiddle.Middle.Size.X/2,0,0) 
+ middle.Finish.Position
-- And its the " Vector3.new(mainMiddle.Middle.Size.X/2,0,0) " part

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local mapsFolder = ReplicatedStorage:WaitForChild("Maps")
local maps = mapsFolder:GetChildren()

local lobby = game.Workspace:WaitForChild("Lobby").Lobby

maxLength = 10
local mainMiddle = mapsFolder.Middle


for i = 1, maxLength do
    local middle = mapsFolder.Middle:Clone()
    middle.PrimaryPart = middle.Start
    middle.Parent = game.Workspace.Track
    middle.Finish.Transparency = 1

    local nextMiddle = game.Workspace.Track:FindFirstChild("Middle2")
    if nextMiddle == nil then
        local newPosition = Vector3.new(lobby.Size.X/2,0,0) + lobby.Position
        middle:MoveTo(newPosition)
    else
        local theMiddle = game.Workspace:FindFirstChild("Middle"..i-1)
        local newPosition = Vector3.new(theMiddle.Size.X/2,0,0) + theMiddle.Position
        middle:MoveTo(newPosition)
    end

    local newMiddle = mainMiddle:Clone()
    wait(1)
    newMiddle.Position = Vector3.new(mainMiddle.Middle.Size.X/2,0,0) + middle.Finish.Position
    newMiddle.Parent = game.Workspace.Track
    newMiddle.Name = "Middle"..i
end