"Would you believe that I got lost in Union Station?"
Wandering up and down the passageway under the train tracks at Union Station, I definitely got lost there. So, I’m putting this out there in case anyone stumbles upon this and ends up in Union Station one day: if you need to take a bus that goes on the “El Monte Busway”, ask for how to get to the J Line. I needed to get on route 487, and I learned that it and the J Line use the same platform. It’s an out-of-the-way extension of the bus terminal at Union, and you have to exit the station east, turn right, then walk all the way down before you see it.
Foothill Transit also operates some of their routes from this extension, and they have a helpful map of the terminal. Just head toward the platform pictured at the bottom.
Oh, and Google Maps is completely clueless about this. Thanks, Google.