The car was kept upright by a pair of gyroscopes, these consisted of a steel wheel rotating in a near vacuum. Emergency props could be deployed by the motorman if the gyroscopes failed!
The car ran on an oval test track at up to 4mp/h and apparently worked perfectly fine with passengers not dumped unceremoniously on the floor!
More about Scherl's gyro monorail cars can be seen here.
|Bogie detail, notice the collector shoe and double flanged wheels|
 "Gyroscopic car in Brooklyn", Electric Railway Journal Vol. XXXV No. 3 (January 1910) p. 116