An infant loses interest in a toy train set every time the train disappears into a long tunnel. this infant must be in piaget's stage of development.

The answer is this infant must be in sensorimotor stage of development of Piaget. As an additional example, Shaina in progress crying because she supposed that her brother disappeared in which when in fact he just put on his Halloween mask. Piaget would say that Shaina is in the sensorimotor stage and has not yet achieved object permanence.