Good observation, but they were used PCCs purchased from the Washington, DC transit system and then significantly modified them for their unique requirements.
Bill
Bill, Although the intial production of the CTA 6000 series by St Louis Car Company used PCC components, the first 200 were built using new parts. 6000-6200
The second order 6201 - 6720 and 1-50 were built from components from the PCCs( new body shell, reconditioned motors trucks,control, motor generators and certain body components.