1. dump or push cart west.
2. Cart is now underwater (probs filling up with water, if you let pond zone fill up to 6/7 like normal).
It's also on valid EW ramp, so is accelerated west no matter what you picked in 1. This takes few steps or few more steps depending on that.
3. Cart is now on valid EW ramp, opposite acceleration. If there isn't a wall to the above, it will exit to above. As such, make sure there is wall to the above.
4. Cart is still on valid EW ramp, accelerating east. In something like dozen steps, it will go on the previous EW ramp.
5. Cart has moved on opposite-oriented EW ramp. Because of checkpoint effect, it is at the very end of it's tile. Because it is moving east, it moves east and up on the stairs.
6. Cart has enough speed to move over the stairs and onto EW ramp, which will proceed to accelerate it west for about dozen steps or something, turning it around.
7. Since stairs count as untracked floor, cart does a small flight and (hopefully there is one, if it arrived this far) hits a wall instead of going back beneath.
8. Cart does it's best Wile E. Coyote impression and falls into water with a splash, generating mist.
9. Cart is on EW ramp, accelerating east....
Also, minecarts are ridiculously weaponizable, with one not quite correctly set up being more dangerous than your average FB.
Still, some links:
Soldiers are overrated (non-military defences presentation) - you'll note that one of the designs looks familiar. (I tend to place two-tile bouncer between two forbidden doors, but to date I harvest plants from surface too much for it to catch any thieves.
Not included in above would be variety of waterguns - there's a lot, so can do a search for them;
here's 1. A watergun also makes for an even more misty mister (I've accidentally watergunned the dining room)
Or machine logic things like precisely calibrated steel spear repeater or slight signal delay so that creature walking over a pressure plate will fall into a hatch.