I put non-sound decoders in mine. I would like to put sound in eventually, but would like to find a way to either make the current motors quieter or re-equip them with quieter motors.
I used either a Digitrax DN166I1D
https://yankeedabbler.com/n-scale-dig-dn166i1d-series-6-board-replacement-dcc-control-dec, or a TCS IMFTA4
https://yankeedabbler.com/1330-tcs-train-control-systems-imfta4-irc-decoder-scale-n-part-745-1330/, both for new Intermountain F units with wired motors. The decoder boards with the feet attached will work, but the feet do not fit, and cannot be used, therefore, getting the version without the feet installed (and needing to be removed) makes sense to me.
The screw terminals on the Intermountain boards are in the same place as the ER board, and using the existing screws will work with no modifications.
The headlight is so short due to the light pipe for the engine reaching as far back as the board. To prevent the light from falling off (happened far too many times), I angled the light down, which seems to prevent it from snapping off randomly.
You will need to run wires from the decoder board to the motor. I was fine soldering directly to the motor, though used as little time on the iron as possible.
Some installation pictures:
Side View
Top View
With shell