The bus, going to Ghaziabad, rammed into the tractor trolley after the driver lost control near Mirzapur village, they said.
Around a dozen passengers in the bus got injured and were admitted to a hospital in Greater Noida. The condition of the bus driver and conductor was stated to be critical, the police said.
Traffic on the Yamuna Expressway was affected for an hour as the rubble of the damaged bus and tractor trolley were removed from the road, they said.