An aggressor nation or extremist group could gain control of critical switches and derail passenger trains, or trains loaded with lethal chemicals.

You know, as director of the CIA, I got an awful lot of intelligence about all the horrible things that could go on across the world.

Well look, CIA is an agency that has to collect intelligence, do operations. We have to take risks and it's important that we take risks and that we know that we have the support of the government and we have the support of the American people in what we're doing.

I think the primary message to the world is that the United States is going to remain the strongest military power in the world.

First, how do we give everyone a fair shot at opportunity and security in this new economy? Second, how do we make technology work for us, and not against us - especially when it comes to solving urgent challenges like climate change? Third, how do we keep America safe and lead the world without becoming its policeman?

Unless the local community signs up, wildlife won't survive. And without wildlife, no one will visit.

I was most eager to see how Trump would respond to the climate-change question.

If managed well, urbanization can create enormous opportunities: allowing innovation and new ideas to emerge, saving energy, land and natural resources, managing climate and the risk of disasters.