You have a couple of issues you need to consider.
- What are you going to park there? It looks like a small station, so maybe a passenger car? You can place straight sections of FastTrack almost next to the wall, but the curve coming off the switch will require more room to allow for overhang of the edge of the car or engine. If you use 40' freight cars, shorty passenger cars, or small diesels, you can probably get away with 1 3/4" center rail to the wall depending on the curve (O-31 may need 2"). Longer equipment will need more room.
- FastTrack switches are set up for 6" from center rail to center rail (C-C) between straight tracks, so they will not work for you unless someone knows how to cut them down. You would need to use something like Atlas or Ross #4 switches, and those can get your parallel tracks down to 3 1/2" C-C.
I expect others will have a better answer, but as a minimum I think you would need maybe 5 1/2" from the center rail of your mainline to the wall, and it looks like you have about 6". You just need to figure out how to make the FastTrack to Ross/Atlas transition somehow to use a #4 switch.
