https://www.youtube.com/watch?v=I9AqRys2nZ8The train and the scenery are fantastic. From this comment and others I've seen that used the magic words "field phone," they evidently used a field phone line for communication between the crews due to the many bridges and tunnels.
"Btw, the train was operated by 7 conductors and 21 technicians. To ensure a stable communication also in the tunnels, a purpose-built cable reel phone line was used."
I hereby pledge to implement a similar system should we ever run a train of similar length over similar bridges and through similar tunnels.
By the way, I'm pleased to say that I have ridden and photographed this meter-gauge line. :-)
-John M