This graphic from the Tribune makes what happened a little clearer:
After the first train passed, they thought it was safe to cross even though the lights and signals were still on and the gate was down. The sound wall blocked them from being able to see the other train approaching.
They clearly shouldn't have crossed until the arms went up and realized that it was unsafe even though there was no arm blocking the sidewalk on that side. Still- I could almost see myself doing the same thing if I was in a hurry. How tragic.
Many of the neighbors are calling for the line to be shut down until additional safety measures are installed. At the very least, I'd bet that UTA will be adding additional signage at the intersections that are blocked by sound walls like this.