I was ALSO over a passing freight train (about 1500 feet out, using Skyreat Antenna boosters), and the same thing happened to me, except I got "signal lost". (I am using Litchi, and use the "verbal reporting"). At one point it said "Battery level 0" (should have been 60%). I was at 140 feet or so--so I felt like I had line of sight. ((I flew the rest of missions higher, and had no more problems)).
I took my fingers off the controls, waited a few seconds, and the train passed. ( I was flying FPV, so I could see where the bird was). The video feed cleared of the pixelation, and I was slowly edging the aircraft (doing a little pivot) in a circle to "reconnect" the antennas, if possible. The signal came back, but it was the second time it had happened over this particular field (I was over a sod farm, near a train track). All I could figure out was maybe the train had wifi or a signal generator on board and it overrode the P4 wifi. (just a guess). Looking at the SD card of the video, the aircraft didn't do anything different. Maybe the train uses a 2.4Ghz signal to communicate with the signal boxes as it goes past.... etc. (Like emergency vehicles can change the traffic lights).
I read a post where you should check to see if your RC controller goes "red" or stays "green" -- excellent advice. If green, you are ok. I was concerned enough about all my recent "disconnects" to take the aircraft up today, and just run it out hard 200 feet in all modes and see what I got. Nothing happened, but I didn't go far. I'll try a little farther next time. Weather and temps have not been cooperating to do as much as I would like.
I think I won't be flying much directly over train tracks--it would kill me to have a train hit it if I lost signal and the P4 ended up on the tracks.
Again, your aircraft may have just landed itself quietly (keeping fingers crossed that you find it).