Picture mountains and a still lake at the bottom of the mountains. The mountains are reflected in the water. Tranquility. This is Tranquility, where it is calm. Tranquility will often depend on conditions, concentration is one.
Equanimity, picture a hurricane, a cyclone. There's a lot of wind, things are really rough, but in the center of the cyclone it is still. This is like equanimity. It doesn't depend on the absence of things happening, the absence of wind, the absence of things contacting the senses, but on a certain amount of wise reaction to whatever comes, it doesn't depend on things not happening.
So Tranquility usually refers to states of mind where the mind has become very still and withdrawn from sense experience. Whereas, equanimity is more the stillness within the impingement. Things arising and passing but there is no liking/disliking and no unwise reaction.