My perpetual-motion minecart grinder has proved functional, but not totally satisfactory: while goblins get flattened very reliably, large stuff has to be ground by piecemeal pushes with little momentum. Loading some boulders into the carts improves momentum on the first collision, but a forgotten beast disrupts the course badly enough that the cargo and thus most of the momentum is lost; when the cart circulates at derail/shotgun speed. Consequently, it looks like the best way to cart large creatures apart is massive load and rollers. Rollers are guaranteed non-derail and a metal cart loaded with native gold weighs ten tons, enough to push aside and keep pinned even demons. If they get pushed, they can't use their building destroyer ability.
Due to some init switches, it seems we got no invaders in autumn for further tests; and only one of the three remaining forgotten beasts could find a path into the grinder - it's in the third cavern level, and the passages in there are badly clogged with bloodthorns. Nonetheless, 1047 dead units, ~900 of which hostiles, all without a military; less than ten fortress members got killed.
PS: another forgotten beast found its way into one of the grinders. It works a bit slowly, but it works. A siege showed up around the change of year, testing the surface bonemill. Trolls are held up by the hatches, which is useful, because they're best at clogging up the system. It's generally prone to get jammed by too many (living) bodies: the cart'll sit on the adjacent square and keeps trying to accelerate into the troll/goblin/beast, providing rather weak pushes. The cart won't completely stop and it keeps doing damage, but it's bound up and other units can walk around it. I'll have to upgrade the minecarts from copper to platinum, that ought to help. The chain of three grinders took out the entire 49 strong siege, but i had to oversee it and switch the surrounding bridges twice to re-path the gang. The foremost trolls might have gotten out of the grinders and into the fort proper otherwise.
Assessment: works, but not optimal. Motorised grinders are better.
While waiting for the siege, i fiddled with the self-regulated minecart repeater and have made the so far smallest spike-capable single-plate design:
╔═▼╗ S═¢S #╔#
╚═▼╝ ╚^▼╝ #╚#
Track Buildings z-1
S - medium friction track stop
¢ - hatch cover linked to output pressure plate ^
The plate gets re-activated 42 steps after turning off, a large enough time window for spikes to be receptive for the next signal. Total repeat time is 145 steps. A few off the optimum, but with such a design, less than 144 total wouldn't be possible anyway.
When the hatch cover is open, the cart cycles through the up/down closed loop to the right, when it closes, it passes over the hatch in the next round, around the left half-loop, over the pressure plate (opening the hatch again) and back into the pit. Takes 4x2 tiles above and at least 3x2 below, possibly more (diagonal movement could cause the cart to jump off to the nw/sw, i haven't tried).