Here you go John, the link to the YLB: YLB - RailSounds Battery Replacement.
The YLB is designed to replace the 9V primary battery that is used in Railsounds applications. It requires one wire to connect to track power, usually under one of the wire nuts in the tender. The YLB works for either conventional or command operation, it will charge down to less than 6VAC on the track. When fully charged (about a minute with track voltage applied), it will support the typical RailSounds board for 8-10 seconds, depending on the sound volume and sounds in use. It seems the whistle normally draws more power, so the charge doesn't last as long.
If your locomotive doesn't have the battery clip, you can buy the cable from Lionel for $3.40. Buy a bunch to minimize the minimum shipping charge. 
This is the same cable that comes with the ERR RailSounds Commander products.
