Trains, in a way, and the key to 18 of this puzzle's black squares. (Ones that begin with the keyword are asterisked. The others are for you to discover.)

Question:

Answer:

SHADOWBOXES

Length:

Similar Crossword Questions:

Row of black squares preceding or following six puzzle answers, thereby completing them
How this puzzle's black squares are arranged
Like aleph in the central black squares of this puzzle's grid
Complete set of 12 shapes formed by this puzzle's black squares
Web concerns ... and based on six familiar names hidden in rows 1, 4, 12 and 15 of this puzzle grid, what the black squares in those rows symbolize
On hold ... or what the seven rows of black squares in this puzzle's grid spell in Morse code
Gather wealth by exploitation ... as hinted at by this puzzle's circled squares?
"Hmmmmm ..." [as hinted at by the three groups of black squares in the middle of the grid]
TREASURE HUNT STEP 2 (include black squares)
Dictionary source for each asterisked clue in this puzzle
Alice Walker novel ... or a hint to 12 squares in this puzzle
Type of gear on which the letters in this puzzle's nine theme squares are arrayed
What black and white squares do on a chessboard
What's revealed by connecting the special squares in this puzzle in order
Secret get-together, or what occurs literally in each of this puzzle's circled squares
What the answers to asterisked clues begin with
Fans of black-and-white squares
Puzzle based on Latin Squares
Like the Oscars ... or the answers to this puzzle's seven asterisked clues?
Geographical features ... or what the circled squares in this puzzle represent
What the three sets of circled squares in this puzzle represent
Phenomenon observed in receding galaxies ... and in the answers to this puzzle's asterisked clues?
Lower septet of black squares in this grid, typographically
Term for each of the 15 asterisked theme answers in this puzzle
Number of times this puzzle's theme word appears diagonally in the grid (to complete the puzzle, shade those words plus the Down answers that begin where they meet)