timeline.addTrack issue

Hi there!

New to Cesium here, I’m looking to add multiple track on the timeline. Unfortunatly it seems only the first one works. My code goes like this:

for ( **Iterate throught time intervales **) {
viewer.timeline.addTrack(
new Cesium.TimeInterval({
start : intervale_start_time,
stop : intervale_end_time
}),
8, // Height
Cesium.Color.BLUE,
Cesium.Color.BLACK
);
}

Am I using it wrong?