I applied online. The process took 3 weeks. I interviewed at Chicago Transit Authority (Chicago, IL) in Nov 2019
Interview
Day 1: Driving Skills video test, Customer Service video test, Reading Comprehension test
If you pass to next step...
Day 2: 2-person interview (and if you pass) background and medical checks. Take home online Fatigue test.
While you do not need a CPL /CDL to start the process, you will be in a holding pattern if you don't have one after a certain point.