There is but it won't be until the start the Seaholm project. And it will just be for pedestrians not cars. Apparently any sort of interference with rail operations is a major deal. And when they are doing Seaholm they plan on moving the rail road a little and so they are waiting to build the pedestrian under thingie when they do that.
A couple other things being that 3rd isn't going to go all the way through. I don't think they are going to take down the old rail bridge over the creek, they are just going to use the pedestrian bridge next to it and maybe later upgrade the old rail trestle to be usable by the new urban rail that might go by there. On Second they are going to build a bridge over the creek, but it isn't really a good place for cars. It will have a road, but only two lanes like the rest of second. The road will end at the power plant, but pedestrians will be able to go under the plant and emerge on the other side near the railroad and Gables.
Edit: Look here at where the people are coming out under the plant: