From TM's 2003 Modern Era Price Guide:
A note on Lionel’s RailSounds (RS): First introduced in 1989, RS has undergone many changes and upgrades since its debut. Unfortunately, Lionel has been inconsistent, and in some cases contradictory, on RS designations. For this edition, we have attempted to identify the various versions listed below. It was impossible to determine the version of RS of some items because disassembly to check the boards could not be accomplished. This applies to items with RS 3 and RS 4. In these cases, we have stated these items have RS 2.5, which is the sound standard for RS 3 and 4, as noted below.
RS 1: Introduced on the 18000 Pennsylvania B-6 Switcher in 1989.
RS 2: Used for a short time in 1994 and 1995. Essentially RS 1 with improved sounds, but no new features.
RS 2.5: Lionel’s current standard for sound, being the TMCC version of 2.0. Early versions do not include Crewtalk or Towercom, which was added in 1997.
RS 3: Sounds are identical to RS 2.5, however the circuit boards are now constructed modularly. Allows CR locomotives to be upgraded to TMCC and RS.
RS 4: Lionel’s recently announced upgrade, with sounds still virtually identical to RS 2.5. This version features smaller circuit boards to be placed in more confined spaces, as well as a more efficient sound amplifier board.
