Tracks cannot be easily shifted. I don't know where you get that idea from.
1. There's not enough railway embankment.
2. Union Station isn't going anywhere.
3. You can't just turn a train on a dime. Curves are very wide and very long.
4. There is BILLIONS of dollars of railway control infrastructure buried under the surface (signals, switch controls, communication, monitoring, you name it. In fact,
Metrolinx just poured 365 in signal infrastructure upgrades in the Union Station Corridor alone.
5. Steel columns need to be braced, or they become very very heavy.
6. Everything would need to be built on piles since the entire railway embankment is fill that was brought in the late 1920s and early 30's just after Union Station opened and they moved all the tracks northward from more or less where the Gardiner stands today to directly behind union station.
Cheaper and faster to replace the Gardiner with a tunnel under the lake.