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Across
  1. 3. signal device (4 letters: HORN)
  2. 5. for reduced speed visibility (6 letters: SPEED)
  3. 8. used for day and night signals (9 letters: SEMAPHORE)
Down
  1. 1. to protect the train's rear (5 letters: LAMP)
  2. 2. Clues
  3. 4. of signal using color, form, and position (7 letters: VISIBLE)
  4. 6. of signal expressed by sounds (5 letters: SOUND)
  5. 7. displayed in low visibility conditions (6 letters: NIGHT)