In the AVE (Spanish High Speed), babies and children under four years of age who are in the same seat as their companion do not have to pay for a ticket. If a cart is carried, it must be folded and stored with the rest of the luggage. You have all the information in: «Choosing the means of transport to travel: Which is the safest?«.
You have all the information in: "Choosing the means of transport to travel: Which is the safest?".