I think the typical 3/4" or 1" horse stall mat, vs the anti fatigue mat, with horse stall mat having more rubber and probably denser might absorb more vibration and provide better sound absorption/deadening.
Were you thinking of laying the mat under the RR ties, or doing away with the RR ties altogether? If keeping them, what were your thoughts for placement of the stall mats? You could cut the stall mats down and lay them under the RR ties to act as a buffer between ground & RR ties. Maybe even doubling up on them if say you use ~2' wide strips to lay the RR ties on. Use remaining sections directly under generator to absorb sound.
Regardless, 15' from neighbors house is pretty close sound wise. You might also have to come up with a standing or hung type of sound deadening barrier between generator & neighbor's side of your property. Lots of ideas depending on your setting... city vs rural, etc.