I looked at this for a while when I was at the site and it is possible for them to extend the river walk into the river 10 - 15 feet to come in line with the Lake Street Bridge structure.
I seems to me that they would need to be 20 feet or more above the tracks to allow for the trains to pass and mechanicals and then I guess they will terrace down to the river. I looked back at the base renderings and model and you more clearly see that the building will sit on the existing parking lot and the upper terrace will span the tracks and then a dramatic wall with stairs leading to the river.
So it will be interesting to see how they handle the end cap design to the north with the Riverbend building. Those building sit at ground level and the train runs in front of them