It seems to me like you could route the rail lines close to the water such that they would only occupy the space underneath the south side stands. In which case, the field could be at ground level.
Then again, they might prefer it elevated to prevent damage due to the occasional flooding. Either way, any extra space would presumably be turned into retail and/or offices. Turning an existing parking lot into a larger garage seems like a better plan. I would speculate that only ~20% of fans come by car anyway, and at 2-3 people per car, that's only about 2,000 cars.