Hello everyone. I have a massive project that I have been working on for about six months now in my spare time. It requires (relatively) high speed item transportation, faster than what hoppers can deliver. Hoppers move an item every four redstone ticks, or 2.5 times per second, which takes, well, way way too long to fill even a single storage minecart. I thought about water, but that is a very limited solution and wouldn't work for my project, which left me with minecarts. Minecarts top out at 8 m/s, and each one with a hopper will pull one item every 4 ticks, which works fine for my purposes, so it's just a matter of putting as many carts in motion on one rail as I can. One cart every two ticks seemed to be the limit for stability.
Then I learned carts can be "stacked" or "smashed the shift together" by pointing 2, 3 or 4 dispensers at one rail and pulsing them simultaneously. This led to my final solution, which is a fast pulser hooked up to a quad dispenser to a sloped powered rail. 4 carts per 2 ticks, or 20 carts a second, all bombing down one rail.
Eventually they more or less normalize graphically, and it appears that this change also has some effects on their behavior. Hills are impossible to traverse reliably before they take on this state, and certain turns tend to result in a jam with a bunch of vibrating immobile carts.
So far I have gathered that when dealing with quad-carts the rails before and after any turn need to be straight. The minimum distance between two turns is two straight rails, an after rail for the first turn, and a before rail for the second turn. When the carts take turns, they seem to slide across the block between the before/after straight rails rather than follow the turn, which means that those blocks are also best left empty. I have also observed the carts "hopping" sideways off of one track (which is completely straight) onto a curved piece of an adjacent track, though I couldn't replicate that behavior at will, and it seemed to only happen before they "stabilized." multi-carts often self-propel (!!) and ignore brakes.
The leading quad-cart in a train is often slightly graphically off still, it may behave differently and need to be pruned from the train somehow before doing anything too advanced.
Mostly when things go wrong with these carts though, you end up with a vibrating mass of carts glued to some particular piece of track until you break enough of them, at which time they fire off in all directions. I have seen quad-carts pass through four blocks and land safely on the other side, repeatedly. Both ones freed from a vibe-blob and those which were functional.
double-carts are their own thing, with their own bugs. I don't know any of them, since quad carts provide twice the bandwidth and I only started down this strange path towards that goal.
I am pretty sure that up to a certain point, how many carts you pump into a single space before setting them in motion will determine the behavior you can expect once in motion, and potentially you could filter them back out into separate streams based on those behaviors, though I don't know where to start on that, and I don't think it's of enough use to pursue. Probably best to find the most functional number per space and keep it at that.
I have no freaking clue what goes on when things or people ride in multi-carts.
At long last, I have arrived at the end of my experimentally-gained insights into the inner workings of minecarts when pushed to their limits. I still have a problem with my device, however. For some reason which I have not been able to isolate, sometimes in certain places on a track which appear to be no different whatsoever from other pieces, the train will just freak out and jam up. It's almost always a corner, though often it happens in the middle of a train. I ask of the community, have any of you experimented with carts like this? Is there a secret well of knowledge related to esoteric minecart behavior? If not, then there should be, so go play with minecarts for a few months and tell me what YOU got out of it. I think somewhere in this pile of weird behaviors is the potential for some very high-speed item movement in vanilla, and probably a huge gold savings thanks to self-propelling.
1.6 btw. I play vanilla, no idea why I wait to update.
edit: I put in images, but they're being dumb, so whatever. Have links instead