The first hint to crack the puzzle "Fast-paced; e.g. a bullet train" is:

It is a word which contains 9 letters

The second hint to crack the puzzle "Fast-paced; e.g. a bullet train" is:

It starts with letter h

The third hint to crack the puzzle "Fast-paced; e.g. a bullet train" is:

It ends with letter d

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The answer for the puzzle "Fast-paced; e.g. a bullet train" is: