Keep tweaking the angles of your ramps until every cart can pass smoothly through the portal and into the exit. The less your ramps move the more consistent the cart path will be. As you can see in the image below, I added some more platforms below my ramps to make them as solid as possible. The carts will fall into the portal in a bunch of different ways, which will more often than not lead to crashes and pile ups. Support the whole thing however you can, and don't forget to attach it to the ceiling with some cables. Below the ramp you will need to make a second platform to lead the falling carts into the pit with the yellow portal, which will open up once the button is hit. They will strike the wall at high speed then fall back down.
Using the anchor points in the upper right corner, make a short suspended ramp that the carts can use to reach the button on the wall. If you are finding that the carts can't reach the portals or the exit, tweak the angle a little to improve their speed.
Do your best to match the bridge angle with the angle of the existing road so the carts don't lose their speed. Use the anchor points above the acid to make short bridges, supported from beneath with triangle arches. To begin, you will want to fill in the missing sections of road at the beginning and end of the level.