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).
See also: now, on, trolley