I don't think the ends of the arms should drop down and touch the ground.
It's been awhile since I've seen actual units but I don't remember seeing them do that.
You could try adding a "leg" to the end of the arm. If it was freely pivoted so it could swing to a vertical position when the arm lowered it could hold up the end of the arm, keeping it parallel to the ground.
When the arm was raised to the vertical the "leg" would swing to the vertical as well and lay along side the "bottom" of the arm.
That make sense?
Won't work with present design. The manufacturer says that there are gears inside the servo motor and they are preset then the housing is glued shut. You have to break the glue seal to adjust the gears and this is what is so tricky. You may break the housing when you try and pry the glued parts off. If the gears were easily accessible, then it would just be a matter of adjusting the gear-teeth cause the servo motor is set and doesn't change.
That's too bad.
And not a good design.
If they are using typical servos the position of the servo is determined by the pulse width of the signal on the input. I use servos on my layout to move semaphore type signals and there are a trim pots (screw driver adjustable) on the circuit board of the servo driver - which is mounted under the table along with the servos. Each trim pot sets the position of the arm for each aspect (red/yellow/green). So if the arm does not stop at the correct position a simple adjustment of the appropriate trim pot and all is well.
In their case rather then trying to adjust the position via "glue" they could have used trim pots with access holes in the housing.
But they may not be using this type of servo - though I know of no other type.
Thanks for sharing.
Here's a direct link to SBSC's web page. They are cheaper if you buy them from Reindeer Pass, but they don't have has large a selection of options:
http://www.sbsignal.com/Large_Scale.html#LSCROSSING