Right then, about ventilation, if we don't put doors on it it'll be fine, as the tunnel ends catch the wind quite well, so there'll always be a some sort of airflow through the tunnel, and having me on a back of a train going through it will help aswell, (I'm 6ft) So I that's all the vents questions answered.
As for what length are the curves, I'd have to go measure that one, I wouldn't like to guess.
Pretoria, if should a fire break at the back of the train, then you should ask yourself why the bloody heck is there a fire in a solid concrete structure?
Invicta, as you predicted, there is steel reinforcing in the roof, and the garden has been put on top of it, and it's had a 1.5tonne mini digger driving over the top of it, so I don't think there is a problem with weight bearing.

Thanks for asking though.
As for plans for steam, it's a case of money on this, as a simple 5" gauge live steamer costs roughly £2,000? Where as a battery run diesel cost around the £700 mark I beleive. I would love to have steam on the line.
Hope I've answered all your questions.
Stew