Model Disappears after using Weld Script?

I am trying to weld a train using a weld script, but it keeps disappearing after pressing run. I have tried these two codes so far.

function weld()
    local parts,last = {}
    local function scan(parent)
        for _,v in pairs(parent:GetChildren()) do
            if (v:IsA("BasePart")) then
                if (last) then
                    local w = Instance.new("Weld")
                    w.Name = ("%s_Weld"):format(v.Name)
                    w.Part0,w.Part1 = last,v
                    w.C0 = last.CFrame:inverse()
                    w.C1 = v.CFrame:inverse()
                    w.Parent = last
                end
                last = v
                table.insert(parts,v)
            end
            scan(v)
        end
    end
    scan(script.Parent)
    for _,v in pairs(parts) do
        v.Anchored = false
    end
end

weld()
script:Remove()

-- Title: Weld script 2.2
-- Date: September 6th 2014
-- Author: Zephyred
--
-- Release notes:
-- * The root instance will be welded too if it's a part
-- * Increased flexibility, each call now creates it's own list of welds so you have more control over it.
--
----------------------------
local P         = script.Parent
----- NO EDITING BELOW -----

-- This function actually creates the weld.
function CreateWeld(x, y)
    local weld = Instance.new("Weld") 
    weld.Part0 = x
    weld.Part1 = y
    weld.C0 = x.CFrame:inverse() * x.CFrame  
    weld.C1 = y.CFrame:inverse() * x.CFrame  
    weld.Parent = x
    return weld
end

-- A recursive function (google it) which traverses all parts in a model to weld them.
function WeldAll(instance, mainPart, list)
    if instance:IsA("BasePart") then
        table.insert(list, CreateWeld(mainPart, instance))
    end
    for _,v in pairs(instance:GetChildren()) do 
        WeldAll(v, mainPart, list)
    end 
end

-- Unanchores all parts in a weld list.
function UnanchorWeldList(weldList)
    for i,v in pairs(weldList) do
        v.Part0.Anchored = false
        v.Part1.Anchored = false
    end
end

-- This is the function you should use if you want to weld a model.
-- Parameters:
-- model = The Model containing the parts.
-- mainPart = The part where all other parts will be welded against. 
-- Returns:
-- A Lua table containing all welds, which can be fed into the UnanchorWeldList function, as demonstrated in the example code.
function WeldAllToPart(model, mainPart)
    local welds = {}
    WeldAll(model, mainPart, welds)
    return welds
end

----- EXAMPLE CODE -----
local weldList1 = WeldAllToPart(P, P.Part) -- Weld a model, returns a list of welds.
UnanchorWeldList(weldList1) -- Unanchores the list of welds given.

script:Destroy() -- Clean up this script.