The London Underground is famous for its iconic posters (as well is its maps, logo, and font). While Twin City Rapid Transit had some posters as well, they did not reach the same level of artistry. (If anyone has a link to an online trove of such local posters, link in the comments).
As an in-class exercise for my Transportation Policy, Planning, and Deployment class, I asked students to create posters for MetroTransit. These are just sketches on a white board, but I hope you can see the possibilities.
The posters can be at stations, on vehicle (exterior/interior) bus shelters, kiosks, anywhere you can post posters. London works with third party institutions (teams, museums, zoos, malls, etc.) to coordinate marketing of transport with the destination.
Following the model of London Transport, posters should sell not only existing customers on new uses for transit they might not normally think of, but also new users. They should try to improve utilization in off-peak times, off-peak days, and so on, when the system is operating well-below capacity and additional demand on existing operations is most valuable.