Hi Stephen,
Soundtraxx makes a Soundcar decoders that supports their plug-in Current Keeper module (just plugs right into the sound car decoder). These decoders have several lighting function outputs as well as sounds, e.g. clickety-clack, flatwheel, random passenger car generator sounds etc. The sounds sync up with the speed by being added to consist as if they were just another locomotive (they also have magnet wand feature where you wave a high powered magnet over the outside of the car where the decoder is installed inside to add them in or to remove them from a train). Of course you need to add your own speaker to get the sounds portion of the decoder.
As stated above, knowing the lighting current draw is first step and comparing against the rated lighting current load supported by the decoder. If the current draw is too high, then you may need to replace with lower current LEDs.
Soundcar Decoder
http://www.soundtraxx.com/dsd/soundcar/soundcar.php
Current Keeper and Magnet Wand Link
http://www.soundtraxx.com/access/wiring.php
Scott K.
Austin, TX