I would like to build my own custom train case to store my N scale locos and rolling stock. I have the coolest idea…my other great passion aside from trains is music, and I’ve decided I want a train case to be housed in the form of an electric guitar case 
I know where to find guitar cases, but what kind of foam should I use to line it and how should it be cut?
For foam, try ReBoxx.
I use foam rubber, the kind like is used for sofa cushions. It’s also similar to the “eggcrate” foam sometimes used in guitar cases, mattress cushions, etc. I think Wal-mart has it in the craft section, or try a fabric/craft shop. I cut mine with a sharp hobby knife. It can be cut with scissors, but is a bit of a challenge for the thicker pieces. Some modelers I’ve seen use a thin sheet of it (say, 1/2" for HO Scale, or 1/4" for N Scale) between layers, then find a thicker sheet (say, 1" or 1 1/4" for N Scale, or 2 or 2 1/2" for HO Scale) and cut out holes in it for each car/loco. In a guitar case, you could “stack” layers of trains maybe 2-3 deep in HO Scale, and maybe 5-6 layers in N Scale.
Brad