GRJ: Thanks to your timely reply, I have been able to Google for the items you referenced but now have another round of stupid questions.
Its now clear that I need to order an ERR RailSounds board for an ALCo 420, in order to have the horn I want. However, I was only able to find a RCDR on a parts list for a powered locomotive listed at the Lionel website. Is there a source for the RCDR board you recommended and the parts needed to connect the two?