A Special Sort of Bus
From Tardis Wiki, the free Doctor Who reference
"A Special Sort of Bus" was a musical piece composed by Murray Gold.
It was used in the Series 4 special Planet of the Dead, and refers to the 200 bus' ability to fly on account of the Tritovorian anti-gravity clamps.
The piece is featured on Murray Gold's soundtrack Doctor Who: Series 4 - The Specials.