Sometimes they are very similar, tranquility and equanimity, but tranquility can often have a kind of pleasant Vedana to it. Equanimity does not have a pleasant Vedana, it's not pleasant, it's not unpleasant, it's very neutral. Tranquility can be very calm, very peaceful, very content, but often has a pleasant Vedana to it. So that is a main difference between the two.
Equanimity also has wisdom with it, or hopefully has wisdom with it. To be true equanimity, it must have wisdom with it. Tranquility may or may not have wisdom, as it may only come from concentration phenomena. When a person is super concentrated, they might get very tranquil and calm, but it doesn't mean they have wisdom. That's another main difference between the two.