Steve,
The MRC CM20,is the older version,with the fan,,newer versions don't have that and a lot of the electronics,in the newer versions also were changed. Unfortunately,if you have your heart set on purchasing,one or two more,,good luck,,,they stopped making them...For whatever reason,,I think the newer ones were the best they ever made...Sometimes they are on E-Bay,,but the seller probably wants an arm and leg for them..
Good Luck,
Cheers,
Frank
the noise i had on my cm20 was the fan i took the back off and reshaped the fan and put it back and no more noise i hope this may help someone i am starting over with n scale i sold all i had in ho scale i would like to use the cm20 for my new layout it will be a small layout maybe 3'by4' double main with two sidings would like to find two cm20's for my new layout thanks steve