my main power is an atlas gp7 chessie. i would like to have a sound decoder that will fit' given small area in the shell. anyone have any experiences? any advice would be helpful
HO or N scale -- they made both.
For HO scale, the decoder is not going to be as much of a problem as finding or making room for the speaker. You need to open it up and see whether or not there's space for a speaker and enclosure.
For N scale, there's only one choice -- MRC.