While wdp4 or wdp 4d gear box is designed to gain maximum speed in less time wdg 4 gearbox are capable of generating maximum torque frequently.
Apart from this may be they don't want to engage wdp 4d as far as possible for banker job.
By the way castle rock dts has large number of wdp 4 and 4d also for banking job in braganza ghat section