This is a really great concept, and the execution is fairly technically sound. I think there are a lot of things that you could do to improve this into something much better, though.
Obviously there are the issues that you have listed in the description. Outside of that, I think there is often very little time to see and click on signs. It also felt like there was a delay before I could attach if I tried to click before I was in range. I think the game needs a much better way of relaying the range to the player. Maybe the sign itself could change color as you get into range, or maybe some sort of visual indicator between the cart and the sign.
It also feels like maybe there could be some obstacles other than just sharp turns to keep it fresh. Perhaps some things swinging onto the track that you need to slow down for? And, I know this would be a lot of work to implement, but perhaps sections where the track is out and you now have to use the rope to swing over the gap.
At any rate, the concept is awesome. The execution is pretty good if flawed. The aesthetics are fantastic. Good job!