I assume you're not confusing an error with the dwarven stupidity of getting tired, drop the magma filled mine cart, and a moron decides to haul the cart to a stockpile instead of its destination (sometime the "place minecart" job beats the "haul to stockpile" one, and sometimes not)? In that case, the "haul to track stop" job picks it up from the stockpile (which may well be just beside the magma loading station, or further away from the destination).
No, I'm sure it was a hauling routes error, and I'm pretty sure it wasn't entirely mine. Dwarves were perpetually reloading dumped iron minecarts back into the bronze minecart on the autodumping trackstop at a route stop with no loading instruction; intent was for the bronze minecart to deliver the iron minecarts QSP-style to be filled with magma or deliver their magma loads by unrelated routes. The offending routes and stops were never intended to load anything; I had sister routes at each end (without track stops) to load the iron minecarts into the (reassigned) bronze minecart, to be taken on unassignment when loaded to a nearby bronze minecart stockpile, from which another bronze minecart stockpile at the far end could take with a wheelbarrow to speed the long commute between the shallow magma consuming level and the deep magma loading facility.
I suspect some sort of indexing error because I did make an erroneous hauling route stop designation. I caught and corrected my error long before any minecarts were applied, but not before I'd designated a few more of the routes necessary to my scheme. Somehow the loading instruction for the non-dumping route stop got applied to the non-loading route stop on the autodumping track stop. It didn't show up on review of the hauling routes interface, though, and various combinations of respecifying route stop loading instructions didn't help (including adding the one I didn't want in hopes that it could be turned off for effect).
Anyway, nice to hear you've got something to work with.
I'm really embarassed that I needed your hint to realize DFHack could do make a trackstop do what I wanted by dumping liquid onto a pressure switch, with mind-numbing simplicity for a single-shot implementation. Resetting such a simple system to fire again, though, seems to need (what I consider) too much tedious designating things for dwarves to do in their own sweet time, and keeping an eye on their progress to designate their next round of necessary tasks (e.g. refilling minecarts with water and getting them back on the trackstops).
I finally did figure out a not-too-complicated system for an automatically regenerating prompt trigger, with complementary make-before-break outputs, activated by DFHack turning on autodumping for one of a pair of portable drains. When one portable drain draws down the level on its pressure switch, it activates a retracting+drawbridge pair to drop a measured aliquot of water to refill and reset the other. With a little care to volume management its possible to keep the switches accessible to add new connections when in the ON or OFF state (basically, the drains live in 3-tile reservoirs beneath their 0-0 pressure switches; drainage leaves 3*2/7 space and 0/0 on the switch ON, and dropping measured 7/7 water fills to 1/7 on the switch OFF).
This system is a little exploity, relying on the portable drain trackstop exploit and the DFHack facility to change autodump behavior of a trackstop (including one in a portable drain).
I have no expertise with posting graphics here, so I'll just offer this crude elevation view of one cell of the needed pair:
wDBd
Sd
GPGd
w water source (beside or above D)
D Drawbridge, on floor, linked to *other* cell pressure switch S
B retracting Bridge, on downstair or open space, linked to *other* cell pressure switch S
S pressure Switch, water level 0-0, on a carved down stair (dig stair, construct floor, build switch, remove floor)
P Portable drain (trackstop+minecart), built with autodump behavior off
G Wall grate (keeps falling water from shoving minecart off trackstop, maybe unnecessary)
d access door for maintenance or connections (adjacent P for easiest use, but this is 2D diagram)
Not shown: levers linked to B,D to prime at startup (or after user error), water supply cutoff, the second cell (same as shown above)
After construction, toggle the priming levers a few times until water stands above both switches S. NOTE that the switches will toggle OFF when their water level rises; if that matters, defer connection to target devices until later.
Then enable autodumping (any direction) on one portable drain P to lower water below its switch S. That switch will toggle ON (again, if that matters to your target controlled device defer its connection until later. That will also flip the other cell's bridges to drop more water onto its other switch S, but that won't hurt anything because it's already wet and OFF. You can disable autodumping as soon as the bottom water level drops to its minimum 5/5.
Then, with first cell drain P autodumping disabled, enable autodumping on the other portable drain P to lower water below its switch S. That switch will toggle ON (target device connections still deferred if it matters), and the first cell's bridges will drop 7 units of water to put 1/7 water above the first cell's switch S, toggling it OFF.
Now it's safe to connect target devices to either switch S; which depends on whether you want the first effective signal that device receives to be ON or OFF. The downstairs allow connection access to switches S; 1/7 water won't stop dwarves from making connections, won't leak out when they come through the door, and won't evaporate while supported by 7/7 water in the tile below.
To toggle both outputs, deactivate (turn autodumping off) the portable drain P under 5/7 water and activate (turn autodumping on) the portable drain P under 7/7 water. The activated drain will quickly suck the water off its switch S, toggling it from OFF to ON, and lower the level in the bottom reservoir to 3*5/7. After a brief delay to flip the other cell's bridges, 7 units of water will fall to fill the 6 units of empty space in its bottom reservoir and leave 1/7 water atop that other switch S, toggling it from ON to OFF and automatically preparing the system for the overseer to toggle back at whim.