red mushroom
n.— «A preliminary investigation also found that the train’s braking system was not defective and that the operator did not press the “red mushroom”—Muni parlance for the bright red button on the operator’s console that activates an emergency brake. The light-rail vehicles are operated with hand controls.» —“Muni train likely speeding before crash” by Rachel Gordon San Francisco Chronicle (California) June 17, 2008. (source: Double-Tongued Dictionary)