Function of characteristic curve of DC motors
There are several factors that make it difficult to develop equations that give motor performance curves.


Nonlinearity of the motor magnetization curve.
Motor mechanical losses: windage and friction
Drive train losses: gear, bearing &amp; wheel friction
Armature reaction effects.


The motor combinations determine the motor voltage. Six motors in series results in 500 volts applied to each motor, two groups of three in series results in 1000 volts, and  three groups of two in series results in 1500 volts. You could consider the three curves to be current vs. speed curves for a single motor at 500, 1000 and 1500 volts.

If you want to draw traction force vs. speed curves, the most accurate results would probably be obtained by using the data from the existing curves.

Re Shape of Current vs Traction Force-Curve:

The part of the Current vs Traction-Force curve that is shown, appears of be fairly linear, but it actually curves upward somewhat showing the effect of the motor torque being proportional to the product of armature current and field strength, where the field strength is proportional to the armature current. If the curve were to be plotted to zero torque a much more quadratic curve would be seen. In addition, the effect of iron saturation works to flatten the curve at high current. I may have time to illustrate this later.

Here is an excerpt from P. B. Harwood "Control of Electric Motors" 1952