The four transit lines all use standard rail gauge, but are otherwise incompatible. Blue Line trains can run on Orange Line tracks, but cannot be used for revenue service because the two lines have different platform heights. Orange Line cars are too long to negotiate the tight turns of the Blue Line. However, this limited compatibility has been taken advantage of. When new Blue Line cars were delivered from Siemens Transportation in 2007, they were tested on the Orange Line after hours, before acceptance for revenue service on the Blue Line. The Red Line is incompatible with the Orange and Blue Lines, as its cars are wider and use yet another platform height; having a train on the Blue or Orange lines run on that line would result in a relatively wide gap between the platform edge and the train. The Green Line is incompatible with the other subway lines due to having a different propulsion system.