I haven's seen this asked yet but sorry if it has. The Bachmann Alco RS-3 featured in the April 2016 Model Railroader sounds interesting. It can be operated on a DC layout with and I phone or I pad. Does anyone know if eventually this technology could be added to straight DC locos as DCC modules are now? Will this become a wave of the future or another flash in the pan?
There is a large topic in the General Discussion section.
http://cs.trains.com/mrr/f/88/t/254909.aspx
In my opinion, this is a one off system. They (the company that actually makes the electronics, not Bachmann) do or are planning on making "decoders" for installation into other locomotives. However, there are not currently sound files for all locomotive types (currently not even an Alco Prime mover for the RS-3).
The first people that should worry about this is people like Railpro as it can do or will be able to do what they do cheaper via software upgrades. The decoder is $75 but that includes you chosing the wiring harness that fits you or the wires to dead rail or keep alive. Andoid app is about to be relesed.