Just to kick this thread off here are my baseboards for Barchester. These have been built for permanent fixing but the same methods could apply for temporary or portable layouts. Only the legs would be different.
The boards are quite simply made of lengths of 2"x1" softwood with supports of the same material every 15" or so. This distance depends a great deal on the type of top surface you are going to use. The thinner and more flexible this surface is the closer the supports will need to be. The supports have holes drilled in them for electric cables to pass through, so helping to keep the wiring a bit tidier. The boards are in 4 foot lengths, each length screwd to a batten on the wall and it's supporting leg of 2"x2" softwood added. The separate lengths are also screwed to each other. As an additional precaution, knowing how clumsy I am, I have added a small steel angle plate to the base of each leg and screwed that to the floor. The stability is first class and the structure would stand a mans weight.
The first picture shows baseboard number one and the second shows all four baseboards together.

Bob, I may have asked this question before on a previous forum, but what is the height of your benchwork?  I ask on the eve of building my permanent structure, as I have plenty of pre-cut 2"x2" cedar of 42" length I am tempted to go with that height.

my baseboards are quite high Tim probably higher than bobs i think your model railway looks better when viewed at nearer eye level than lower like a lot of people do. the reason they were built higher in the first place was because of the level of the garden railway section but after doing it i was pleased with the height.
cheers Brian

Thanks, Brian.  I won't go over 3'6" in height  but I do agree that the less of a looking down perspective the better.  I am six foot tall and my daughter will soon catch me up so … :D

Also - from a selfish point if view - that saves me cutting ten pieces of timber to a shorter length      

I've added my vote to your poll topic,Tim.
In it I've put mine at 46" (which they are) but if I had 42" timber, I'd use that and be perfectly happy.

Thanks again, Jeff - I replied to your post on the poll thread :D

I did something similar with my baseboards but instead of the legs going down vertical to the floor, I angled mine back to the point where the wall joins the floor & attached them to a vertical post screwed to the wall.
This way, I can add shelving & stop people kicking the legs at the front. This technique I used on 3 walls as they were 19mm chipboard walls anchored to steel framing.
 The other wall being brick, I bolted the back horizontal piece of the baseboard framing to the wall in about 3 places ( 20 ft wall) , 3 vertical legs bolted to the wall & one vertical leg at the front & this was also braced horizontally on the floor to stop any movement. Diagonal braces here as well. I ended up with about 76  feet of shelving - 2 levels. One shelf 16" wide & the other about 20" & still not enough storage space !!

Lift up with 4 tracks at the doorway. Basic level is 44" high.

I like to view and operate Barchester from a sitting position in a rolling computer chair so that dictates the height of my baseboards and gives me an eye level view.