Extreme Physics Lag during Playtesting

Help and Feedback Game Design Support
#1

Here’s my train (running on physics) during a ‘run’ and a ‘playtest’:
(Sorry about the bad quality, my character is not laggy at all)
Running the game:

Playtesting game:

Why is this lag occuring and how an I prevent it?
Please ask if you need any more details, and all help is appreciated.

Additions:
Yes, I have checked for malicious plugins and everyone I have seems legitimate. The game is not Team Create so no plugins from friends have interfered.

#2 
For I, part in pairs(train:GetDescendants()) do
     If part:IsA(“BasePart”) then 
          part:SetNetworkOwner(nil)
     End
End

Try doing this

#3

I tried running this when the train loads but this has not fixed the issue. Seems like it might be something related though. Thanks.

#4


Here is what is happening when the owner outlines are on.
The train should be anchored to the base that moves away.