While others have gone the single decoder route on this locomotive, I would not advise to do so.
My green 'Frankie' came with two type 1, #55020, decoders. These are not the most sophisticated nor competent at fine motor control, but I found that to maintain basically similar speeds, I needed a higher CV2 on the loco than on the tender drive. CV2 is the start voltage at speed step 1. The locomotive, due its rather complex circuit board, has a higher start voltage requirement than the basic, almost non-existent decoder interface circuit board in the tender. I was able to get matched speeds with CV2 programming.
After much playing around setting things up, I gutted the loco and will fit a Massoth 'LS' to the loco and an 'L' to the tender. The two type 1 decoders are now seeing service in a #20761 Spremberg and as a direct decoder on a LGB 'shorty' sound board.