During the late night hours, the agent at many Chicago Transit Authority rapid transit stations would go off duty. Before leaving, the agent would unlock the turnstile and turn on the "pay on train" lights that were upstairs or downstairs on the platform.
The lights were not only for the passengers, but also so the conductor knew to collect the fare or transfer from those boarding at that station. They carried a coin changer and a hole punch as tools to help them do so, in between making their usual public address announcements.
Most conductors were fast enough to be sure to collect fares from everyone on the overnight 2-car trains, even with the stops being very close together. If no one already on the train wanted to get off at a given station, some would, instead of using their automatic door controls, use the emergency control to only open one door for the use of boarding passengers, and would collect fares before they could even sit down.
On southbound Evanston Express trains, which only run during rush hour, the conductor would also collect a surcharge during the express portion of the trip, between the Howard and Belmont stations. The surcharge could either be paid by giving 25 cents to the agent at one of the Evanston stations and getting a receipt to give to the conductor, or by paying the conductor 50 cents in cash. In the morning, heading toward the Loop, these 6-car trains were completely packed, but the conductors always managed to collect the receipts or quarters from everyone.
By the late 1990s, the CTA had completely switched their rail service to one-person operation, eliminating conductors by using automated announcements and having the motorman open and shut the doors. They had also entirely switched to fare payment through the use of the Transit Card, with a stored-value magnetic stripe, putting vending machines in every station, and had eliminated the surcharge on the Evanston Express. Progress had eliminated the folksy "pay on train" option in the third-largest city in the United States.