The Transparency property of a Trail sets how transparent the segments of the trail are over the trail’s Trail/Lifetime. This value is a DataType/NumberSequence, meaning it can be a static value or can change throughout the lifetime of the trail segments.

The values in the NumberSequence can be any number, but the effective range of transparency is 0 (completely opaque) to 1 (completely see-through). The Transparency property for TrailEffects defaults to 0.5.