Does it sound thin by itself (it shouldn't) or in conjunction with the other pickups? If it is thin by itself, I say there is something wrong with the pickup or wiring. If it is thin with the other pickups, it is out of phase.
It is probably out of phase with the middle pickup. Try reversing the black and white wires
Yep, wrong wiring or bad solder joint. Does it have 2 or 3 wires coming from the pickup? With 3 wires it is a SSL-5T.
Can you measure the DCR? Should be round 13k ohms.
Second this. It's a fat pickup compared to a vintage single coil but compared to a hot bridge humbucker it will sound thin.What exactly is it replacing? You mentioned a rail pickup. If it's replacing something like a Hot Rails, then your amp's settings may not be ideal for the SSL-5.
What exactly is it replacing? You mentioned a rail pickup. If it's replacing something like a Hot Rails, then your amp's settings may not be ideal for the SSL-5.
Have you moved your second tone control from the middle to the bridge pickup?
Seems it might be dounce to the objective definition of a fat sound.
Will it be fatter than the ice pick stock bridge? Yup.
Will it come anywhere near a "Fat sounding" (think Tone Zone) humbucker? Nope.