It handled it pretty well, but I kept it covered all the time and just uncovered it for the very short flight to capture the train. There were some small drops on the drone after the flight, but nothing critical as it was too less to really get the mavic wet.
Yes, it looks like the little one in the front took most of the work, but I'm pretty sure that it wouldn't be able to pull the full train alone. My assumption was that the two G 4/5 did most of the work and the G 3/4 was just for show. Based on the announcement, it should have been a two heading train with 1x RhB G 4/5 and 1x G RhB 3/4.
Yes it looks just a little weird. Today I have seen the clip again on a larger screen than my laptop, and I can actually see coming a small amount of steam from the locomotive No. two just before the cab. Ok, it's also down the hill where you made the lovely clip so maybe the two "big" just had a break.