Al-Qaida has already said it's focused on public transport, according to this video featuring an al-Qaida spokesman, the American Adam Gadahn.

VOA: standard.2010.04.01

Their bombs did not explode. Analyst Jonathan Wood with the security firm Control Risks in London, says public transport systems are an attractive target.

VOA: standard.2010.04.01

But analysts say public transport is hard to police.

VOA: standard.2010.04.01