What does it cost to ride the train

$3 for the day

It's not free. $3/$2 round trip. Nice ride.

If you get the pass for the botanical garden, aquarium and zoo it's free.

As of last week, it was $3 adult / $2 child to ride the train that runs between the Zoo and the Aquarium/Botanical Gardens (the Rio Line). Their website says it's the same price for the Train that circles the Zoo (the Thunderbird Express).