Help Location 24VAC common wire on Trane Air Handler

Hi, I have a Trane Air Handler (2tc3f60) and am tryign to connect a new wifi thermostat to it original thermostat only has 4 wires (red green, yellow and white) new one says it requires 24vac common wire in order to function I read that this should be on the transformer in the air handler I found a blue wire not connected at the air handler but have no idea which is the 24vac common