Using the pinned topic link (http://www.clickthai.de/Land/Karten/download.html) I have some good maps to go by.

Does anyone have a legend for the map? What do the red, lighter red, and yellow lines represent? train, road, gravel road?

Go here http://www.clickthai.de/Map/index.html and click the 'legend' link, this defines all the symbols, unfortunately it doesn't say what the road type are.

Looking at the maps, they seem to show only major roads, certainly they don't show dirt roads.

Looks like:-

Red = major roads / highways

Yellow = smaller roads (although 'small' is subjective, our pretty big road isn't shown)

Black dotted = railways

Purple = county boundaries

Light blue = rivers / khlongs etc

There are also pretty good maps on google maps, overlaying the satellite images shows you the dirt roads that the map doesn't indicate. IMHO a better option for local navigation.

Single digit numbers are major highways, Double digit are normally four lane or at least good roads. Triple digit are good roads and when you get to the four digit numbers you can expect about anything.