Hey, thanks for hopping on our server and playing with us tonight. Overall people seemed to really like it. I like your idea of lowering the spawn time for attackers a bit, since I think that would help prevent stalemates and make those checkpoints worth more.
Here are my simple suggestions:
1. There's an overlay/decal arrow on the floor that's placed under the track. I think it would look better to the side, even if it means coming up with a different overlay design.
2. The upper area at the end of each side needs a little more cover/detail. It's very wide open right now and an excellent sniper will completely control that. I think one row of equipment up there would really help and would give engineers a bit of protection as well (on offense and defense).
3. Reward players for going into the water at middle somehow. This could involve increasing the health down there and making the other health in the area smaller or something like that. Right now it's primarily a spot for spies to recharge, but it'd be cool to see it used more.
4. The first forward spawn seems a little complex, IIRC. It's not a big deal since your arrows are excellent, but I remember there being at least 2 turns, a ramp, and 2 doors to go through. Perhaps make the spawn room itself bigger, make that first door glass, and move the spawn itself further back so it feels less cramped. I don't remember it in detail, so I could be completely wrong on this.
Other than that it was great. Keep in mind that you shouldn't base much of the balance on watching our game. We have an interesting mix of excellent players (including one of the top demomen in TF2), people that just screw around (we had a pyro that was just airblasting and axing the whole time), and people that do things in really unorthodox ways (like when I play offensive engineer). So just temper your opinion of watching us play against that.
EDIT: Someone suggested a cart model that looked the same forward and back. I think this could be a good idea. The arrow on the cart hud is hard to figure out on the fly, so if you can get the colored cart working somehow that would be a great improvement. Otherwise consider some sort of integrated lights around the track that show which way the cart is currently moving. Don't do it if they'd look cheesy, but it might help the gameplay slightly.