My layout is in my attic in SC. In the summer it gets to 108 degrees and in winter as low as 38 degrees, though those are the extremes. Within one day the temperature swing is about 20-25 degrees.
I haven't had any problems with track joints. I use Gargraves and purposely did not push the sections completely over the track pins to a flush joint. I left about 1/8 inch gaps to allow for expansion. My scenery is almost all styrofoam (lightness is the priority here). I have avoided using plaster so I don't get expansion/contraction cracking. I have a lift-out section which requires a tiny adjustment (thicker shims) in the winter months.
I only have one command locomotive (TMCC) and have run it in both winter and summer. A through the wall A/C cools the room in summer and I have two small electric heaters to warm it in winter. My proto 1 locomotive has run fine as well. Like the previous poster, I run the A/C or heat for about 1/2 hour to cool or heat the room as needed.
Overall, I'd say that you won't have any issues with conventional equipment. I don't have enough experience with command equipment to know what the effects might be.
I would watch the humidity. The heat, humidity and salt air (only 8 miles inland from the ocean) accelerate rust. I wish I could store my trains inside the house, but there just isn't any room.