This one is for the signal experts. Last night I was down by the tracks, the track is a main line with a passing siding. The lights are a vertical three light mast, constant yellow then two reds below, have seen this alot and know what this one means. But as the train came closer, within half a mile, the yellow light began to flash. It continued flashing until the lead engine passed then it went red. I could see down to the next set of signals at the end of the siding and the same thing occurred. I waited around for a short while then noted the top light then went yellow again. Along came another train and this time the light stayed sonstant yellow until the lead engine passed, then red, so did the next light. My question is what does the yellow flashing light signify, why did the light stay yellow then begin flashing as the train gets very close. Any reason that it did not do this for the second one? Both trains were coal trains, both same company, CP Rail. Thanks. Ron..