Dual-mode bus

Several of the examples listed below involve the use of dual-mode buses to travel through a tunnel on electric overhead power.

Many modern trolleybuses are equipped with auxiliary propulsion systems, either using a small diesel engine or battery power, allowing movement away from the overhead wires, called "off-wire" movement, but such vehicles are generally not considered to be dual-mode buses if their off-wire capability is very limited.

Examples include the fleet of about 300 trolleybuses in San Francisco[1] and the trolleybuses used on a 2005-opened system in Rome, Italy,[2] which are capable of running on battery power only for short distances or short periods of time before needing recharging.

[2] Dual mode trolleybuses in a number of Chinese cities can operate significant distances (8 to 10 km) off-wire on battery power.

[3] Media related to Dual-mode buses at Wikimedia Commons

Boston Neoplan DMA-460LF dual-mode trolleybus, operating in diesel mode (with its trolley poles lowered)
The Downtown Seattle Transit Tunnel was served exclusively by dual-mode buses from its opening in 1990 until 2004.
A dual-mode bus in Ploiești, Romania, 2010.