For radio communication, CTA uses some of the traditional police "ten" codes, and has many "ten" codes of its own.

10-1 Receiving poorly
10-2 Receiving well
10-3 Stop transmitting
10-4 Message received and understood
10-7 Unit out of service
10-8 Unit in service
10-10 Police needs assistance
10-20 What is your location?
10-21 Call by telephone
10-25 Dirty vehicle
10-31 ATC bypass operation authorized
10-32 Stem bypass operation authorized
10-33 PCR bypass operation authorized
10-40 Radio check
10-41 Unauthorized switch
10-42 Abandoned bus
10-49 Check farebox
10-50 Make special service check
10-51 Go to location and assist as appropriate
10-52 Manpower shortage, runs held in (buses)
10-53 Use appropriate service restoration technique
10-55 Inspect all viaducts in district, check sewer covers
10-56 Bus equipment shortage, runs held in
10-57 Rail car shortage
10-58 Trip annulled (rail)
10-59 Salt over/underpasses, bridge approaches, etc.
10-60 Check employee
10-61 Employee injured
10-62 Employee sick
10-63 Other person sick or injured
10-65 Switch to alternate channel
10-68 Repeat message
10-71 Collision of CTA vehicle with fixed object
10-72 Collision of CTA vehicle with a person
10-73 Collision of CTA vehicle and other vehicle
10-74 Collision of CTA vehicles
10-75 Derailment
10-77 Have crew make announcement
10-80 Fire
10-82 Police assistance required
10-83 Throwing at bus or train
10-84 Assist with fare dispute
10-86 Disturbance in progress
10-87 If train is standing in station, remain standing
10-87 Train between stations stop outside next station
10-88 Alarm: bus, train or station
10-90 Bomb threat
10-91 Weapon threat
10-99 Dire emergency, stop all other radio transmissions

Information is from the "Scanner Master Chicagoland Pocket Guide", edited by Richard A. I. Carlson and Ted Moran.