Around hte world underground systems are known to run part of the way at ground level in roofless stretches and somehow the elements (rain, etc.) are no obstacle
Because, as I said, they will have used carriages designed for both under- and overground networks. Rolling stock designed for such a purpose is more expensive than single-purpose trains (purely under- or purely overground.)
Re terrorism, it is not likley ot go away anytime soon[/quote]
It's not likely to arrive any time soon either. And the location under the highest threat from terrorism is nothing so banal as a public transport tunnel.
Tube systems have above-ground (elevated) monorail network: cheaper, safer and more dynamic and modern looking than trams and buses.
Yes, the L-train system in many American systems can be praised for its "modern" look. And could never be criticised for rendering whole swathes of real estate as worthless due to the noise, reduced light levels, pollution and inherent crime. A perfect solution for the centre of downtown Warsaw...
Next time I go for a stroll along the leafy avenues of kamienicy in Powisle, I'll try to imagine a screeching rattling overhead train and think of you.