Is the PWM rail of a 4pin pwm Computer fan Positive or negative
It's a PWM control signal, a digital signal input to the fan. It's also not a rail (in electrical engineering/electronics terminology, rail refers to a single voltage supply). Consequently, to refer to it as "positive or negative" is incorrect: it's a digital signal, driven by the motherboard, that switches between 0V and some positive voltage, and the pulse width controls the fan speed.

Nominally, the control signal is an open-collector output, 25kHz PWM signal. The fan is expected to pullup to 3.3V or 5V. If you want to run the fan flat-out at all times, you need not connect the PWM input, as it will be pulled high by the fan.

For more detail information, refer to the 4-wire PWM-controlled fan spec, courtesy of Intel.