MTH subways were not designed for grades. You would need an extra power unit with-in the 6 car train in some cases. Might want to think about that issue first. You will also go through more traction tires as well. Not worth the trouble.
Most operators of existing layouts attempt to force an underground line as an afterthought in which in the majority of cases will be too low to properly view. I guess one could possibly extend the legs?
Just an FYI.
