Egad! Sherlock Hemlock here, the worlds greatest detective.
And I believe the letter following "W" is...

"X" marks the spot.
"X" marks the crossroads.
Two straight lines crossing
Over in the middle.
That's the letter you must see to
Find the treasure or the clue,
Or the X-ray machine that's
Looking right inside of you.
"X" is the letter without which
One could not do.

"X" marks the spot.
"X" stands for danger.
Two straight lines crossing
Over in the middle
Saying "Here's the railroad crossing"
And sometimes it means there's pirates
Or at worse, it means there's poison
And in case these thoughts of "X"
Are vexing you
"X" stands for "xylophone," too