Variants of a mold are catalogued separately, so each one has its own sets and minifigs. Check these too if something you expect is missing here.

• 160ac01 Plate Special 2 x 4 with Train Coupler Open with Magnet Long Cylinder Red this part is a sub-part of it
• 160ac02 Plate Special 2 x 4 with Train Coupler Open with Magnet Long Cylinder Blue this part is a sub-part of it
• 737ac01 Plate Special 2 x 4 with Train Coupler Open with Magnet Short Cylinder Red this part is a sub-part of it
• 737ac02 Plate Special 2 x 4 with Train Coupler Open with Magnet Short Cylinder Blue this part is a sub-part of it
• 737b Plate Special 2 x 4 with Train Coupler, Open for Magnet [Flat] this part is a mold of it
• upn0272a Magnet Coupling, Train - Long Cylinder (8 mm) this part is a pair of it
• upn0272b Magnet Coupling, Train - Short Cylinder (6 mm) this part is a pair of it