WDM-2A is applied
to those that were re-fitted with
air brakes (most of these
therefore have dual braking
capability), while WDM-2B is
applied...
more... to more recent locos built
with air brakes as the original
equipment (these very rarely
have vacuum braking capability
in addition, especially if they have
been rebuilt by Golden Rock).
(However, in the past, before the
widespread use of air-brakes, a
few modified versions with a low
short hood at one end like the
WDS-6 were also classified
WDM-2A.)
source: IRFCA