Avoid any form of duck under, lift up, lift out, drop down, swing bridge, etc. I had them, never again! It will get old, just like you.
Have the stair case enter in the center of the room permitting 4 x 8 sheet goods to pass. Full size steps with no turn.
Electrical sub panel for that area at head of stair case, fully accessible.
Sprayed in ISO foam full depth floor, walls & ceiling. Embed conduit for all anticipated circuits. Get your rough in inspections completed prior to spraying insulation.
Advantech 4 x 8 sheet material for platform decking. Cover with Homosote.
If 95% plus gas heat be sure the condensation drip tube will not freeze. If freeze protection is not possible go 85% conventional. This one point can be painful.
Whatever flooring you choose install it first then protect it with overlapping sheets of 6 or 8 mil poly cloth until plaster is mostly finished on your layout.
Understand, any free standing RR building will be the most expensive location for your trains. Extra taxes, gas and electric for ever plus initial construction cost, just a fact of life. I had that at my last home and avoided that problem by getting a 2000 sq. ft. full basement w 8' ceiling this time around.