A bit of research in "numbers behind the scene" for minecarts:
the speed required for "derailing" (ignoring an unforced corner and continuing in a straight line) really is >50k, i.e. anything over 50.000, no matter how little. There'd been a recent edit on the wiki page that claimed it was an unknown number. Not so: a cart going at precisely 50.000 speed takes an unforced corner, a cart going at 50.020 ignores the corner. Anyone who needs any higher degree of precision than that can do the research themselves
The shotgun effect (cart expels its cargo/occupant, which keeps going at the speed and in the direction of the pre-shot cart) requires a speed of >55k. To the best of my tallying ability, that's a similarly sharp cutoff - no shot at precisely 55.000, jettisoned cargo at 55.020.
Since carts also loose their cargo when a standing cart is collision-accelerated to >55k (cargo "flies out" at zero speed plus the shotgun-typical random element), the effect is not really tied to a speed but to a
speed change of the mentioned magnitude. Point in case: i fabricated frontal collisions between a willow cart containing a kaolinite (275 urists weight) and a date palm wood cart with a hematite boulder (550 urists weight), so that the heavier pushed the lighter cart, resulting in an elastic collision conserving momentum, i.e. the willow-with-kaolinite cart moved off at
twice the date-with-hematite (and also its own) pre-collision speed. At a pre-collision speed of a mere 18.340, the kaolinite boulder was expelled:
18.340 pre-collision speed
36.680 post-collision speed in the opposite direction
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55020 speed change, enough for the shotgun effect.
The speed was achieved by simultaneously dropping the carts off hatch covers onto track ramps and then guiding each over one medium and two low-friction track stops, and four tiles of normal track. Since they started accelerating in the middle of the ramp, they only received five steps of ramp acceleration, of which one was neutralised by the checkpoint effect, giving them a speed of 19550 before going over enough track stops and normal track to eat away 1210 speed.
As far as i can tell, hatches (and presumably grates and floor bars) opening under a cart directly force the cart into the centre of the tile before dropping it and "softly" set its speed to zero. Regardless of pre-drop subtile position and movement vector, carts always dropped onto the middle of the ramp tile underneath a hatch. Carts going at shotgun-able speed over an opening hatch dropped into the hole without letting go of their cargo. Carts behave differently if "dropped" by letting them fly over a hole and bump into a wall.
PS: speed cutoffs added to "numbers behind the scene"; i also fiddled with the sub-tile explanations some more, cf
here.
BTWPS: it's theoretically possible to build a minecart shotgun using no source of acceleration other than dwarven pushes. Shots would travel between both push stations, at under 20.000 speed, and on almost all attempts there wouldn't be a shot at all - carts would need to be pushed almost simultaneously _and_ about 50% of all collisions would auto-fail because the lighter cart would push the heavier one. But dwarven pushes give 20000 speed and - see above - 18340 are enough!