get a hot wire cutter?

The wire cutter will help making nice curves for the tunnel opening. You could cut and sand smooth. I found Bosch jigsaw blades on Amazon for cutting foam. T313AW3 6".

The sculpting tool or engraving tool would be used for the mortar lines.

The pencil does nicely for stone and such. I watched a video on youtube using the pencil technique. It looked great. I will be making a six foot viaduct soon.  I haven't figured out if I'd have the patience to do brick with a pencil.

My vote is ...pencil works.

This entire bridge was done with 2" foam, coated with plaster, and then the bricks were individually carved.  That's Harry Henning's work, it looks very impressive in person.

gunrunnerjohn.... other than plaster what else can one use to seal the pores in foam so paint will look smooth?

If you cut it with a saw, I'm not sure what else you might use, it is pretty rough.  I'd consider maybe some paint that was oil based might do a bit of melting and seal it, but I've never tried it.  The issue I have with hot wire cutters is the speed, they're pretty slow.  I have one, and I use it for small stuff.

I have used a ULINE hot knife for many years. If you are going to work with Styrofoam, a hot knife or cutter is essential. It cuts clean, quick, with no mess.

As much as I like the clean of hot tools, the smell is not so nice.