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 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).