now you're on the trolley
Now you understand what I mean or how to do this. A: "So this piece slots in here, and we connect these two wires together, right?" B: "Yeah, now you're on the trolley!"
Now you're on the trolley
Now you catch on. “Trolley” refers to the streetcars that predated buses and subways in major cities. To flounder around to the answer to a question or how to perform some sort of procedure and then to come up with the right answer was the equivalent of getting on a trolley that's on the right track (as in track of streetcar rails).