I was doing random things today and was messing with the Furnace cart. I found out that by placing one Mine track, then looking at that track while holding the right mouse button with Furnace carts in your hand, it causes many to spawn on that one track, which leads to the glitch.

The Glitch:
If you look up at the middle of the carts and hold right mouse button (Making one of them move), they will all fly off together and play the rolling minecart sound repeatedly and rapidly, and it moves like a ghost train.

I did look to see if there were any duplicates. Maybe not hard enough.