Several readers have asked about this: all Expo and Blue Line trains will be leaving from platform 2 at the 7th/Metro Center station in downtown Los Angeles.
Announcements are being made in the station. Another easy way to tell which train is an Expo Line train and which is a Blue Line train is to look at the signs on the train.
If it says “La Cienega” it’s an Expo Line train.
If it says “Del Amo, Willow or Long Beach” it’s a Blue Line train.