Are there any circumstances in which an infant would require a ticket?

If three or more infants accompany one "Adult" or "Child", a Child ticket is required from the third "Infant" onward.
A Child ticket is also required with reserved seat tickets.
However, with seating above the rank of reserved seating in ordinary cars, Child fare tickets for higher-rank trains (Green tickets, GranClass tickets, and sleeper tickets) are the same
price as adult tickets