What is the function of Duplo part# 38507 RAIL ACCESSORY, NO. 4
It's an interactive piece that makes the train perform a cool(ish) function when it rolls over it.


The train has a sensor on the bottom that reads the piece and does the deed.


When the train reads this red one... it stops moving, which is sorta coolish  for a jealous older sibling maybe ;).


Watch 3 videos explaining all the cool possibilities.  (1), (2), (3).