The minecart was teleported through 13 and 16 tile at this scheme. !NOTICE! Its not 13 an 16 tile from ramp (start of the experiment), its 13 ground tile next behind the first fortification and the 16 tile next behind the second fortification! After the third jumper/fortification was not teleportations at all.
Yeah, this is what I was describing in my post above. When the cart is moving > 1 tile per tick, it 'skips' over some tiles, with the game never appearing to reconcile what would happen if the cart was at that coordinate. Toady can fix that, but it's tricky. The alternative is to cap the cart speed at 1 tile per tick, which the current cap appears to be close to, and which I suspect was his intention, so the behavior may go away. The reason you see no teleportations at all after the 3rd jump is the speed of the cart came down to 1/1, so the cart never travels more than a tile each tick.
If Toady doesn't fix it, it has some useful implications as in my testing, the tunneling effect is pretty reliable in terms of where it happens over repeated runs. At a minimum, it solves the railgun pathing issue. If you don't want anything to path back up your railgun line, simply put a wall at the end at precisely the right location. The gun will still work and nobody can path in. It could also eliminate some of your collision damage by putting the high-speed line in a completely sealed run relying on tunneling to have it emerge through the wall of the workshop. Because it can't tunnel through more than one tile, if you time it out right, just put the stop immediately next to the wall and it should catch it every time.
It may also work as a switching mechanism. If you want a given cart to skip over a track stop, accelerate it at just the right point so it tunnels over the stop. Unaccelerated carts would stop.
And the implications aren't just for carts but for everything else. In my testing, several of those struck by the cart tunneled through a wall and were found at the map edge. I even increased the height of the wall to make sure that it wasn't due to them flying over it and periodically they would still tunnel through. I don't know how to utilize this for anything productive as it's completely unpredictable, but I suspect Toady will need to fix it - it's going to be intolerable if stuff keeps tunneling through walls, flying across the map, or getting embedded in walls and such. So, I'm not counting on keeping this, but it's fun for now.