It takes an iron man to play with a toy iron horse.
If I want a rough cut, I gouge with my steak knife. If I want a smother cut, I slice with my steak knife. If I want an even cut I use my hot wire tool with a custom shaped wire. It all works. If you bust through, just hot glue a patch on the bottom and continue. I have not tried any on top of plywood, but that would give more strength if you have to crawl on top of it for working. I do have stair treads under the stream bed in the canyon so I can walk right in to work on it.
Have fun, foam makes great scenery.
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.