Perhaps before worrying too much about what you actually make the baseboard out of, you might like to look at what goes under the layout and think some how you are planning to "display" your fabulous streetscape story.
You have appear to have already decided that it will be in 2 bits - you also need to think about how to support and join these bits.
Things to consider:
support leg type - attached or seperate
joining type and positive location - so that the bits go together first time every time
Wiring across bits - each bit selfcontained/jumper wires/plugs
Lighting - inbuilt or seperately attached after layout is together
Have a look here for some inspiration on legs and presentation:
for a lightweight baseboard, don't write off layered foamcore - see here
The rest of the Simplicity Sidings thread should also give you some more inspiration in the baseboard and tracks in pavement.
If you are prepared to go a bit narrower than 20" wide to just 14" wide under you could use a similar arrangement to "Yallah"(see picture in presentation thread). It uses a framework of aluminium square section and Queblok fastenings at each corner with a laminated foamcore as the baseboard. This type of construction ensures that the hinges and track stay accurately alligned and layout remains dimensionally stable at the joints. The Brooklyn 3am layout uses the same aluminium and laminated foamcore construction methods.
It stores as a 2' by 4' by 2'6" cube when not in use but folds out and latches into an 8' long by 10" ldepp layout with a 20" high viewing apperture with enclosed lighting low-votage halogen rig in the 5" deep valance above the layout. Thie height to the the top valance could be increased slightly for your layout to match your building heights. I suggest the reduced width as it should allow you to walk the layout through a "standard doorway". At 24" wide Yallah is an easy fit trough most doorways.
- Caution - always watch out for the doorknobs.
At the reduced width your layout would store at 3' 2'6" by probably 2'6" high cube and would fold out to a visible area of 72" by 14" with the track on the drop leaves taking up about another 12" depth when folded down along the back of the viewed area. The only reservation I have about this style of layout is that the 14" width that would allow you to fit through a standard doorway is that 14" may be insufficient to fit in your buildings, a footpath, at least a lane of roadway and the train track. However if you are prpared to run wider you could finish up with a 72" by 18" viewable footprint.
The Yallah design also has folding dropleaves that allow a return track circle behind the layout when it is set up. This allows for continuous running of your train/tram. Yallah was designed with 15" radius curves to allow 6-coupled steam locomotives to be run. If you limit yourself to small 4-coupled steam locos and small diesel/electrical locos then you should be easily able to go sub 12" radius for your return circle. At 15" depth, If you pushed the track to the front of the layout, it should be relatively easy to arrange for the train to disappear off the street through either a narrow alley between the buildings or through a gateway. The folded drop leaves add about 2" to the folded thickness of the stored block.
Of course there is the seperate bits option where everything travels as seperate bits. Just some thoughts - whether they are useful of not is up to you. Keep us posted with your thoughts and I'm surre that the crew here can sort out the "trainy bits" while your streetscape saga develops. The things to be considered above will help you dodge most of the land mines of exhibiting "Belle Lane". The questions need to be asked, but the answers do not have to be always difficult.