Chicago Transit Authority makes it easy for people and businesses to set up a digital screen feed of transit information

Some places do this.  Some places did this but didn't keep the application updated.  Others implement it at least in transit stations and bus stops with real-time updated signage screens too.  But many people are still driven to create their own displays, because the local transit agency doesn't provide such an application.

Chicago Transit Authority does. (Brochures webpage.)  I argue these are forms of media developed by government agencies, but they often aren't thinking that way.  Event notices and other information can be delivered by similar systems.