We just received this tip from reader Chris Gabriel:

Just now a bus caught fire in the engine compartment heading uptown on 1st ave at 56th st. Acrid smoke started pouring into the passenger compartment at the back near where I was sitting in the last row. The driver stopped the bus and got everyone off quickly. There were flames visible under the bus before what looked like a fire countermeasure was deployed. I was impressed by a quick response from the MTA and NYFD.