I would suggest to spend some contemplative time in Armstrong"s layout design book. One aspect he develops quite well is that there is nothing more expensive than a free standing train room. Every facet is additional.
I know this in real life because I did it before I read his book.
I had to provide a dedicated HVAC system, I had no benefit of thermal spill over of a shared wall or ceiling. Constant turning the temperature up and down did not help stabilize the wood. Even with R-35 in the walls and R-58 in the ceiling, recovery from big temp differences was always a delay when I went out to the train room. I had to dress up in harsh weather to visit the train room. Bug season was always a pain. Rain was another frequent thing to contend with. Just stepping into the train room in the middle of a sleepless night did not happen. It got to a point that many times that I did not look forward to the effort to go out in the train room. I had to fight with zoning code department to pull it off. It banged my real estate taxes real hard, which of course became a permanent annual expense. I grew to resent the set up.
That prompted a purchase of a long ranch home. An open dry basement 1970 sq ft with an 8' ceiling that already had the benefit of a temperature radiance from the living space above. No zoning issue. I can walk into the train room at all times of the day any day of the year, dressed "come as you are". Just wish I had this setup years ago.
Separate train room? Nope! Done that, got the tee shirt. Never again.