Typically a track magnet. You'd connect a magnetic reed switch to the circuit board (may be included), hang the switch on the bottom of the loco somewhere between the wheels, and put the magnet on the track. I think the "convention" is the whistle on the engineer's side (left side) and the bell on the right, but it's your railroad.
You can also trigger it with a function output of an R/C control system, but if you're going that route, you're better served getting a full sound system from MyLocoSound, Dallee, or Phoenix.
Later,
K
has anyone had any experience with ram track sound systems, how for instance is the whistle or bell activated, Thanks Bill
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