Lionel has supplied several diesel units with "Railsounds", and their instructions for proper hookup and operation leave something to be desired. Here's how to do so, courtesy of Chuck Furlong. Railsounds requires use of the Lionel # 65906 Control Button.

The # 65906 control button has one red and one black wire coming from it. The installation is simple. Wire it in series with the transformer variable post and the track. That is, run one wire to the 7-15V post and the other wire to the hot rail (NOT the Base rail) of your layout. The base post is wired to the base (ground) rail normally. Your choice of connecting the red wire or the black wire to the transformer will determine whether the button will operate the horn or the bell. It doesn't matter much, because the button's function will also be changed by turning the sound-unit around on the track. To operate BOTH the horn and the bell without changing connections, you will need two buttons. Connect the red wire of one to the track hot rail, the red wire of the other to the 7-15V post of your transformer. Then, connect the two black wires to each other AND TO NOTHING ELSE! Again, which button does what will be determined by the direction the sound-unit is facing on the track.

If you also have the Lionel 5904 button as well, wire this in series between the transformer and the 5906 Railsounds button. The 5904 has a small red slide switch which automatically reverses activation of bell or whistle.

Essentially, all these buttons do is add a DC offset voltage to the AC current already being sent to the track, just like the Gilbert control for the 314AW steamer, and like the horn control on a Lionel transformer. The difference is that the 314AW and old Lionel whistle/horn systems were not sensitive to the polarity of the DC offset. Lionel's more sophisticated electronic Railsounds system uses polarity to separate the horn and bell functions. That's why you need two buttons, (one for negative offset, another for positive offset) to work both features conveniently when using a Gilbert transformer.

A Lionel transformer that already has a horn control (such as a ZW) will operate the Railsounds unit but again, only the bell or the horn. To operate both, you would wire the Railsounds button to offset the track voltage opposite to the effect the internal horn control has. Why didn't they put two buttons in with the horn unit? Apparently, Lionel assumes that most Railsounds operators will be using a Lionel transformer, and will need only one button to complete their horn/bell controls!

Thanks go to Chuck Furlong for supplying this information for posting.

Lionel Railsounds buttons are sometimes available from Port Lines, although they have become increasingly difficult to obtain so it is best to check availability before you order. Lionel is no longer manufacturing these controls.