Most of the ULT files were provided by one or two Railpro users who have access to working locomotives. On that note, of anyone else has access and can get recordings, I encourage you to talk to Tim Ring and see what he needs. We all love new recordings, but most of us are unable to record them.
Speaking of ULT sounds, I encourage you to compare them to the top DCC offerings. Listen to Loksound's 16-645 E3, then listen to Railpro's. To my ear the Railpro sound is superior and sounds much more natural.
Kevin