I have 2 Atlas n scale B23-7 diesels. One is the #49708 Santa Fe and the other is #49731 ATSF(kodachrome). The 49708 has a black motor case and yellow lights on the dc board and the 49731 has a gray motor case and orange lights on the dc board. I installed an MRC 1811 dcc sound decoder in the 49708 and an MRC 1641 dcc decoder in the 49731. They are both programmed for the maximum high speed voltage (255 & 32). The 49708 runs very fast (nearly leaving the track), but the 49731 runs about 1/4 to 1/3 as fast. Do these engines have different speed motors (fast, scale, slow) or is the 1641 dcc decoder not giving maximum voltage to the motor. I want to consist these. No help from MRC or Atlas yet. Can anyone help with this ? Thank you.