So I'm making a train and basically it worked fine as 2 car until yesterday when "Upgrading ROBLOX" Appeared on my launch and then it just got all slow and then when it got to like above 30 speed it just LAGGED LIKE HELL like what the heck did roblox do to the physics?! Please help!
Yeah, the old Legacy Physics might possibly be deprecated (there's still hope; check if you can use the legacy ones). I'm sure that the old joints and things do not work properly with the newer physics. I recommend rebuilding it with the new joints and physics system.
Here's a useful article to help with both your confusion and your knowledge: https://blog.roblox.com/2015/10/roblox-introduces-new-physics/