Blue styrofoam vs. Pink styrofoam vs. White styrofoam - Model Railroader Magazine

 Extruded Polystyrene comes in two flavors Blue and Pink.  Blue is produced by Dow and Pink is produced by Owens Corning. 

Lowes Sells Dow products;

• Residential sheating available in .5", .75", and 1"
• Wallmate available in 1.5", and 2"

Home Depot and Menards sell Owens Corning products;

• Foamular 150 available in .5", 1", 1.5", 2"
• Foamular 250 available in .75", 1", 1.5", 2"

Ok there are probably other flavors but you get the idea.  2" is the strongest and will support nearly a mans weight at 16" OC, the average man weighing 160-180 lbs.  2" is what I would use as subgrade details are easily carved out of 2" material without cutting completely through.

There are special foam adhesives, straight liquid nails generally eats through foam as its solvent based.  There are recipes that are foam safe.  Also Expanded Polystyrene, the white beaded stuff isn't a good bet for structural strength, however its easily carved so makes a good mountain.

Expanded polystyrene can easily be cut with a standard TS and blade, I haven't seen any blade degradation after using it for foam.  There is also surfoam tool, hot wire knife and traditional saw/knife.