EDIT2: Modified this post to remove rambling and make it more pure science /EDIT2
Alright, my first question is to make sure that I haven't completely misunderstood something: Has anyone gotten an impulse elevator over 8zs to work properly? I can get short ones to work. It's when you start getting some serious columns going that it goes horribly wrong. I've tried to keep up with all the minecart science threads but I just haven't seen one that goes into the issues I've had. If there is one please tell me and I'll move this experimentation and testing over to that thread.
I use Phoebius, btw.
This all started when a 90z impulse elevator just refuses to work after the first 8-10Zs by derailing into the wall and rolling back (and rocking horsing if you don't have an escape hatch on the impulse ramp), and I've set out to figure out why.
Warning, this is SERIOUSLY long, and I go through my testing methodology so that those with enough interest can poke holes/offer advice for anything I wasn't thorough enough with. Skip to the end if you want to just review my findings.
EDIT: Sorry, I rambled a bit. This was written out in pieces over the course of hours, and at one point when I was overtired. I'll try to clean it up later, when I'm not pressed for time.[/EDIT]
Alright, let's start with some basics from my initial minecart science center. Version is current, using LNP R22. I should mention I have no idea how, in DFHack, to check mincart speeds, friction, or anything similar. All 'facts' are observations, usually repeatable. I will mention if they were not (some of this is happening while I write this, other items are things that caused said testing).
A few local defintions:
Impulse Turn: An S shaped set of ramps where you derail onto the first one for speed, and then go up the second one in a curve.
De-Skidder: An impulse ramp along the main track after leaving a series of impulse turns to get the cart to continue down the rails. The cart is still using floor skid (or possible ceiling/wall, not sure) at this time, which leads to the usage of a Re-Railer.
Re-Railer: The final impulse ramp after an impulse turn series to get the cart back to following the tracks.
1z Test:Cart is pushed in from west, no impulse ramps as speed kickers prior to the impulse turn. It hits a carved impulse ramp facing NE, then turns upwards on a NW.

When it enters the upper level, it derails. I should say it really more skids than derails, but it acts like floor friction, thus, I need a rerailer impulse ramp. Further testing later proves that this cart actually is skidding across the ceiling from a derail.

Multiple tests were performed to confirm that the cart constantly stopped at the same location with no variance. The rerailer was placed here and the cart was able to continue on without issue, including un-walled turns, confirming it was re-railed.
Multi-z Test:Next test was to perform a multi-z test. Please forgive the trees growing in the middle of the track, this test required some fluid mechanics to clear the cart after a few mishaps with railway carvings and a cart that was causing voluminous injuries while I attempted to remove it. The aquifer was tapped to free the cart. Ignore the brush, mud messes up tracks badly and I rebuilt it after I took the screenshots. The ramps are in the right places even if they're not in the pics.
It was used a few times while I performed various failed tests. I'll discuss those after.
In the Multi-Z test, I was having a problem even with a Re-Railer. The construction was simple enough, a series of impulse turns up 5 z-Levels:

But I was having problems sticking the landing. I finally realized I needed a deskidder AND a Re-Railer:

The first ramp just kind of skidded the cart along at a more controlled pace, and then the Re-Railer could get to work afterwards.
Now, to try to combat needing this, I tried a few things:
First, I built the final ramp as a single up ramp up, trying to burn off some speed from the impulse. It's not enough.
Next, I finally deduced that maybe I just had too much speed in the impulse elevator so that the cart was launching itself after. You'll notice some constructed walls at +5. I was wondering if the straightened ramp was the problem, so I tested a level down after a turn. No significant difference, still needed dual impulses for continued travel, but with a minor variation.
Now, some testing was interrupted by a giant woodpecker, so I just went back and reran the track (after some rebuilding), to confirm that the impulse elevator was still functional (with a load of granite in it) with the 'extra' up ramp at the tail. It worked exactly the same with the granite in the cart, with no spillage. On the impulse elevator the cart is moving ~100,000 Urists, at one tic/square. Once it hits the top, it stops nearly dead on the deskidder, and takes ~6 tics to move to the next square, slows to the re-railer, and then moves at ~25,000 Urists through the turns (and makes the wallless turn on the rails) to the track stop in the lower right, load intact.
So, I'm up to learning two things now about impulse elevators. First, you need to rerail the cart after departing the elevator. Second, you need to skip a few impulses, or speed gets out of hand.
Begin Megaproject, the magma forge ore dumper with cart return. I didn't THINK it was a mega-project at the time, and then my miners took ages digging the system out. And then the headaches began.

After an automated stockpile dump at a low track stop and a bit of maneuvers, getting back up needs to happen in 3 stages. The first stage will be getting back over my magma forge, because it's directly below the area I want to use to elevator up past the 3rd and 2nd caverns. Then after some maneuvers it goes up a huge # of Zs, to right under the first cavern layer. Manevuer from there to an unbroken passage up through the first cavern layer, then to the cart return area. I'll document the cart return elsewhere.
So first it's a short hop up, a mere 6 Z-levels, similar to the one in the minecart testing area, detailed above in the multi-z test. Without the extra speed burnoff straight ramp, it was worse. Two deskidders and then finally a rerailer would work. No joy. This didn't look good for Woody. If he'd only gone to the police, this would never have... wait, wrong show.
I unbuilt the impulse ramps, recarved the track, and removed the impulse ramp on Z+4 and just carved it as a straight connector on ramped ground towards Z+5. I got a rollback, didn't make it to z+5. Repeat at +5, putting the impulse back at +4... and still no good, another rollback. Keep in mind this was not a directly up ramp after the impulse turn, but a flat track to simply connect it to the next curved ramp up.
So why did it work before in the testing facility with one 'unimpulsed' level but not here? Out of curiousity I observed speed for the two systems on entry. Entry speed to this elevator is 6 tics/square, roughly 16,500 Urists. Entry speed at the testing center was... 5 tics/square, with a 6 in front, so figure 19,000 Urists? A mere 3,000 might make the difference but it's unlikely. I triple check all the ramps, yep, nothing random. So, it's most likely not speed. The impulse turn system brings the cart into the 100k range for speed almost instantly, it's just not enough of a variation.
The only signficant design difference is that at the testing facility, it goes up two ramps in a row, once on the curve and then immediately into another straight ramp to the next Z-level. Wondering if having the ramps in a row make the difference, I flatten the ramp connecting Z+4's upward turn to Z+5's straight ramp to Z+6. I'm expecting a flying derail, but I'm curious.
Yup. It's in flight... I think. It hit the flat track at 1tic/square, same as up the elevator, basically skipping it. There's no speed loss there. Then it hit the ramp going to Z+6 and rolled backwards in 3 tics. Okay, so that won't work. A little redesign and I use a two-ramp in a row system again between z+4 and z+6, similar to the multi-Z design final riser.
That's the trick. I'm now getting nearly the exact same results as I did in the testing center. So the problem at z+4 possibly wasn't that it didn't have the speed, but that I was catching flight, landing on the far ramp, stopping dead, and then doing the rollback. More testing.
Back in the minecart testing facility...
I want to find out where an impulse elevator gets enough speed to not need a booster on a particular level. I start with two tests, a Z+1 ramp and a Z+2 ramp. With a single impulse turn it's got enough speed from a push to go up Z+2. Interestingly, a dual impulse turn still only has enough to push past one single straight, to Z+3.
In the images below you'll see the test setup. The bright blue dots are where the carts would stop continuing up the continous ramps after the impulse turn. The ramps continued up further for curiousity, as you can see at z+3 for the single z, but were unnecessary. They would then slide back down, and on the impulse pop up into the 'safety trap' to avoid rocking horses.

So, my original understanding was poor. On an impulse elevator a single Z of Impulse Turns are enough to start a critical speed concern that amplifies with continued impulse turns. My next test is to start playing with the Z+1 for consistency. It was consistent in where it would stop, but I'm trying to get a consistent, non-derailing impulse elevator going. For that, I need to know what it was going to do on Z+2 after the track straightened.
The Z+2 test showed that I still have derailment at Z+2 off a single impulse turn + immediate up ramp, as the cart will travel 4 blocks down and then stall out.

Now, I'm pretty sure a re-railer would work here, but I can't seem to get it to stick on its own consistently. There are two places I'm thinking about taking this testing. The first is entry speed, but that would really only matter to a single Z Impulse Riser, so not necessary useful for my final goal. The other is to see how bad things get over multiple Zs where I skip every other Impulse Riser. The second one is going to take some time to rebuild out into my existing Impulse Elevator. It was built when I thought I knew what as going on from the wiki.
Drop ChuteFor those who are curious about how the drop chute works:
It drops from Z:111 straight down to Z:-14. There is no ore loss on landing, but it does take a little trickery to get the cart going again. The cart needs to land on a ramp, and terminal velocity for a wooden minecart in freefall is 9 squares/5 tics, so it's possible to skip the ramp's impulse, so you need a Multi-Z catch ramp. When the cart hits the first ramp at speeds where it wants to skip the tile because of free fall speeds, it'll move along the track towards the next tile, and stop dead if you don't.
The Yellow X tracks the initial drop chute hole for the ramp for reference.

After such a high speed catch I installed a high then a medium track stop to keep the thing from derailing at down-ramps and bouncing off walls as it maneuvers towards the low friction track stop automatic dumper. From there it continues into the rising impulse system.
TL;DR
More testing needs to be done, but it would appear a long Z impulse elevator doesn't work as described in the wiki. The cart builds up far too much speed during the elevator and derails itself along the way, smashing into a wall and then going into a rocking horse if you haven't built safety outlets at the impulse side.
A single Z impulse riser has enough energy to push a cart up 2 Zs worth of ramps, and still derails when it goes to flat track. Rerailers will help with this process but an impulse elevator is meant to be tight. Testing needs to continue to be done with how often to skip an impulse riser to allow for more controlled elevation of the cart.
Short hops (6-8 Zs) can be managed by using a de-skidder and a re-railer impulse ramp to get the cart back on track (pardon the pun) with a standard impulse elevator. Make sure your last Z is a straight up ramp (non-impulse risered) from the last curve to burn off speed or you have to do more work than is necessary.
Drop Chutes work fine, no spillage, with large items like rocks and with water in a cart. I haven't tested multi-Z carts with smaller items, like sandbags, to see if small things fall out.