I got a couple of the diesel ones (MRC-1636) for the Atlas locomotives. I put on in a GP38, one in an SD35 and fried the third. They're okay - sounds are generic. Some are good, some bad and a couple horrible.
MRC is also doing a drop-in for the KATO SD70M/AC4400; same type thing except the speaker is under the back section of the board instead of mounted on top.
Tim Nixon