ctdabomb, I'm really glad you're picking up this project again! It's turning out nicely. Here's some feedback based on your latest version, regarding pacing and gameplay:
In the thin tunnel with slime beneath the floor, after the first chamber, there is a straightaway that feels disproportionately long. I think this could be made a bit shorter, perhaps the same length as the other straightaway at the end of the same tunnel. Think about pacing: try to make all the features of the track evenly spaced out along the road.
The width of the road in this thin tunnel is fine how it is - please do not make it any wider! Once the walls of the tunnel eventually get covered in pipes and machinery, it will seem very claustrophobic and tight, which I think is needed in at least one section of the track.
I think it would fun for the thin tunnel to emerge into the large tunnel through a hole in the wall of the large tunnel, so the racers drop down onto the road. This would make the road a bit more dynamic, and give a purpose to the second straightaway in the thin tunnel (it would be leading up to a drop). I did a quick illustration to show what I mean, as well as some ideas for the art pass later on (sorry it isn't coloured - perhaps I'll colour it later). The red arrow indicates the path the karts will take.
The two roads here connect at a pretty sharp and abrupt angle, which makes a change from the rounded curves usually seen in the game. Due to the height difference, the player won't be able to see the road ahead until they have already began falling, so it will probably be necessary to add a directional sign on the wall opposite the opening, indicating to the player to turn left.
I honestly feel that the width of the metal mesh bridge over the slime in the large tunnel could be made thinner, AND have the barriers removed.
If the player is given a message beforehand of where the track is going to go (and in this case, they are, as they can plainly see the bridge ahead of them while driving), you can make the track quite thin and don't need to give it barriers, especially if it goes in a straight line.
There currently isn't much of a chance for the player to prepare themselves for the jump at the end of the large tunnel. You could possibly add a short straightaway before the jump, so the player can line themselves up with the jump properly. The same goes for the tunnel immediately after the jump - it could be a bit longer. Maybe the tunnel after the jump could be angled very steeply downwards as well? To add more vertical variation to the track, and keep the player on their toes.
In the next chamber with the archways in the distance: You don't need guardrails here either! Just bank the road into the curves, and make sure you've angled the road and adjusted the elevation so that the player can see what is coming next and plan ahead their turns - this is basically done already. In my original sketches for this track (which is cool to see that you're still sticking to!), this part of the road was supposed to be in a massive, bottomless chasm. Coming into this section should give the player vertigo. You can add to this effect by taking away the track's guardrails, and by making it as thin as absolutely possible (I think its current width is good, definitely don't make it wider) while keeping a reasonable difficultly level. Just make sure the track ahead is visible to the player so they can see what's coming next, and that the road is smooth.
Finally, I think the last chamber of the track before the final tunnel could be made a little bit more interesting. You could do this by enlarging the chamber, making the straightaway much longer, and then splitting the straightaway up into three or four seperate platforms, a bit like stepping-stones. To see what I mean, there is a section exactly like this in the track Deep Sea Driving in
Crash Nitro Kart (the epitome of racing track design!):
https://youtu.be/VITbxUdeY74?t=38. This will work best if the track is still gently curving to the left, so the player still has to think about turning slightly.
Good luck with the track.
When it comes time to do the art pass, I would love to give more concept art and help flesh out the scenery of the track. I personally hope that you focus on making the road as drivable as possible, and rely on this to stop people from driving off the edge, rather than using guardrails or barriers. I really want to see more challenging tracks in SuperTuxKart, and hope that this track will be a trendsetter in that regard.