  We analyze the optical properties of Radio-Loud quasars along the Main
Sequence (MS) of quasars. A sample of 355 quasars selected on the basis of
radio detection was obtained by cross-matching the FIRST survey at 20cm and the
SDSS DR12 spectroscopic survey. We consider the nature of powerful emission at
the high-FeII end of the MS. At variance with the classical radio-loud sources
which are located in the Population B domain of the MS optical plane, we found
evidence indicating a thermal origin of the radio emission of the highly
accreting quasars of Population A.
