Thanks Bruce! Yours and a few others on MLS are the reason I decided to go with the ladder roadbed for my expansion, but I don't see where you mention the spacing between the spacers. I see the spacers themselves are 2x2. From the pictures I would guess at 10" apart? Also, I see you put your switches on some sturdy roadbed, do you have a picture of what the underside looks like? That is, did you taper the ends of the branch line so they meet with the mainline ladder rails?
On the POC spacers are placed about every 8". This figure was determined by the fact that the roadbed is attached to cross members on my raised benchwork every 16". This puts every other spacer directly over a cross member. 8" has worked out quite well for maintaing rigidity. Since very little material is used for a spacer there's no need to skimp.
I use 6" but it is probably over-kill on the straights. However, I find it helps in keeping things lined up on the curves. I should add that I am using 1/2" thick Composi-Deck fascia boards from Lowe's cut into 1-1/2" wide strips for the rails of the ladder. Composi-Deck is much like Trex, and I think quite a bit less flexible than an HDPE product. TuffBoard (which I think is HDPE?) is not available. So I probably have more of a challenge forming the curves than if you have access to HDPE.
Another down-side of Composi-Deck is that is does have a tendency to "creep" under certain conditions. By that I mean it sometime sags or bows over time. Can't quite figure out why it does it on some spans but not on others. I'm sure the Texas heat has something to do with it. But I find if it is well secured (two screws in about one in three spacers, and especially where a "rail" splice occurs) seems to help. So thats a long winded way of saying "I'm sticking with 6 in. spacing but its probably my problem!"
For completeness, I use Composi-Deck balustrades (1-1/4 x 1-1/2 if I recall correctly) and space them far enough for 2" PVC pipe supports. The total width comes out to just under the width of the track ties. The color of Composi-Deck is a dirty gray so it blends in well if it shows. I started with 1-1/2" PVC supports but found it a bit too narrow. Honestly, I can't remember exactly why it bothered me enough to change, but I did!