2023 Medallion Hunt Clue #6

I always hope that it stays hidden ‘til Sunday

My guess is, by now, that you’ve realized the runway

That it sits on is the Soo Line, but where on the trail?

If you look back, the letters of each line tell a tale

I’ve left a few lines without a capital letter

If you put them together, they spell a better

Way of finding which bridge and which water way hides

The resting place of this ultimate prize

Once you’ve found out the name, go down under with care

And from underneath, reach up into the square