If you are talking about an actual seat, like a detachable car seat, you better check to see if it can even be brought in, and if that is the case, you might need a ticket, then, because of the space you'll need.

If you are talking about one of those carriers that you use to carry the baby up close to you, against your chest, you won't have any problem.

My kids are long since grown (one actually has a 9 month old I'll be hauling to a game or two, I hope), but they never needed tickets until they were about 5 to 7 years old - which did coincide with the start of the Weaver Era, ironically, but it is a ticket taker call, and they are all very flexible.