The 57004 may be used as an onboard receiver for higher power locos. This has been done by many 1:29/1:32 scale modelers. It is done with track power or battery power.
The 57004 is sound capable. It does not function at this time. It will work after the USB adapter becomes avaliable.
The 57000 SD or SS are avaliable now with sound. Output is limited to 5 amps. The sounds are generic and limited, but are acceptable to many people.
Tom