Nin is the number of teeth on the smaller gear (attached to the handle) while Nout is the number of teeth on the larger gear (attached to the train tracks).
Nin = 6
Nout = 12
Angular Velocity Ratio: mV = -Nin/Nout = -0.5
Torque Ratio / Mechanical Advantage: mT = -Nout/Nin = -2
The negative sign is used for external gears. The train tracks turn at half the angular velocity of the handle but with twice the torque. This makes sense, considering that the tracks must support the weight of the train.
The above graph shows the absolute values of the angular and linear velocity of the train for a given absolute angular velocity of the handle.