Offsetting time and position of several layers relative to preceding layer

Hello -

Pardon the newbie questions, but I've keyframed 'Black Solid' moving along the x axis and basically would like to duplicate the layer (perhaps with a new color) several times so that each 'new layer' follows the previous layer and offsets itself a certain amount of pixels...say 20px for example.

I found a simple expression that I applied to the first dup'd layer and it works fine but I can't figure out where to tell it to offset 20px to the left of the main layer.

thisComp.layer("Black Solid 1").transform.position.valueAtTime(time -.1);

I suppose I'm trying for a cascading effect + offset.

Thanks!

gj