The SDH104 series of Digitrax SoundFX decoders come "ready to run" with pre-loaded sounds appropriate for the locomotive for which they are designed. All SFX series of decoders come preloaded with generic steam or diesel sounds at the factory. With the addition of the Digitrax PR2 programmer and SoundLoader utility, all Digitrax SoundFX decoders now may be programmed with your own custom sounds. SoundFX decoders support the playable whistle feature with F2 key on DT400 throttles. SFX decoders include a 28mm speaker with harness.