Negative tractive force values within ORTSMaxTractiveForceCurves aren't considered

[Carlo Santucci]

OR moves to zero any calculated tractive force value < 0.
However there are cases where negative tractive force values exist. E.g. in old triphase locomotives equipped with asynchronous motors, when the speed becomes higher than the sync speed (usually when running downhill), the motors start braking, acting as generators, and therefore their tractive force is negative.

[Carlo Santucci (carlosanit1)]

Fixed in x.4079. If there are negative tractive force values within ORTSMaxTractiveForceCurves and the computed tractive force is negative, the latter is not clamped to 0.