I have a newer HO-scale Rivarossi 4-8-4 with an NMRA 8-pin socket underneath the sand dome.  Seems like a no-brainer, but none of the HO scale decoders fit, no way.  I contacted Hornby, who markets Rivarossi now, and they re-directed me to a hobby shop in Ohio.  Those folks said to us an N-scale decoder.  I used a Digitrax DN163-PS.  It fits nicely, and has the amperage capacity for the loco.  However, I smoked the first one.  I had first bought an HO decoder since it’s an HO locomotive, and needed to take off the boiler shell.  The factory wire joints weren’t quite as craftsman-like as they could have been, and some of the leads had strands sticking out that had not at all been soldered.  Testing the bare chassis with a VOM (you can use a digital multimeter) went fine, no shorts.  In the process of testing the bare chassis with the decoder, then moving things a bit to put the boiler shell back on, the wires moved, shorted, and toasted the board.  So, the lesson is to examine these joints to the NMRA socket with a magnifier to check for stray strands.