zythe_ requested feedback on this map a while ago, but I've been unable to write up anything until now. While this version of the map does have part of another prior stage in it, it is not complete so I will be focusing on the final stage.
While I've made and tested five payload maps in my time at TF2maps and was a judge for the Payload Checklist contest, I don't consider myself an expert at the mode. In fact, I consider asymmetrical mapping to be my weakness so keep that in mind! Use your best judgement to draw conclusions from what I've written; there's a lot of potential here and I'd hate to get in the way if you have other thoughts.
When playing this and the previous version I got the sense that the overall scale is correct, but fights happen at longer ranges than they should be. The track length and size of areas seem right, but it's quite barren in some places and leads to long sightlines and fights made up of chip damage rather than decisive kills. The A->B outside bit feels better than the rest since teams are pushed though the highground where the combat feels like it's happening at the right distance.
I like the idea and general layout of the opening area, but there's something missing from the execution. There's no meaningful difference between the spawn exits for Blu, and a lot of the time teams just seemed to be able to pile onto the cart without issue. Yes, there's the big rock in the middle that blocks some sightlines, but all entrances dump into the same empty space that isn't interesting to attack or defend. I'd take a look at Barn Blitz, it seems like it's the most similar to what you're trying to do here.
I know it would completely break the transition you've set up coming from the last stage, but I'd like to see the track zigzag more than it does now, starting more to the right than it does now (from a top-down view, with the cart's starting direction being forwards). Shifting that over, moving the big rock, and reworking the left exit for blu into some kind of platform or building that takes up more space I think would help make the opening more interesting. Keep in mind I tend to lean towards chokier payload design! Here's a sketch of a possible idea:
In general, I think it would be beneficial to do some more things like this along much of the track; a lot of the spaces to hold and push don't interact with the track much, and pulling cover and height closer to the track might help.
There's some nasty sightlines for attackers entering first, and it plays strangely because of it. The thing that feels like it should be a flank isn't because you have to cross the sightlines to get there, and the other flank is basically a shortcut that comes mostly from behind the direction defenders are coming from. Overall, I just don't know where to set up once the front line falls and defenders pull back into the point room. There's a few options here; emphasize the high ground by pushing defenders to the outside of the room and making the fight diagonal between that and the shortcut, or pull defense back and cut the inside edge route so teams have to push the main doors and flank instead of slipping in behind. Since this stage is 3 points, I'd argue the first should be somewhat difficult, then second be easier, then final being hard.
The transition to second is fine, but the point itself isn't great. It's very flat, very open, and I have no idea what's supposed to be happening. I'd cut the shortcut for defenders and push both teams to round the corner following the track; this would cut sightlines and turn the fight into an interesting push down a hallway, with a flank in from the side. Teams and enter and exit from all four sides of the room right now, cutting that down to three is what I'm proposing. It gives Red the option to either hold forward where they do now or back at the end by the curve after the point, instead of being scattered all over the place. This would make it more predictable for attackers having to push in, too.
Final and it's transitions are neat, though there's a lot of empty space the cart needs to cross once it's in final. Seems like you're trying to do a sort of Badwater-like thing, but I'm not entirely sure it's working. There's so much dead space between Red's left side defenses and the track that it's too hard to hold in my eyes. Not sure what else to say really; it's the final for a multistage map, it should be harder for Blu than it is now.
If you've got questions about my feedback, let me know! I look forwards to seeing this move forwards.