Okay let me tell of my one bad experience with a similar situation. It was NOT on a helix, but rather a tight turn on my old Atlas 'Central Midland' layout.
It occurred on a section of track that is the same as where this CSX train w fuel tender is setting,..
The train had come up a relatively steep grade (double mainline) and makes a 270 degree turn to go over that bridge.
Now imagine that I have my BRAND NEW Broadway limited 2-6-6-4 class A Norfork/Western articulated steamer sitting there instead of those CSX diesels. I had just gotten that engine in the mail the day before,...my first articulate loco & first sound loco.
I had a friend coming over to the house to pick me up to go to some event/party/whatever,..I've forgotten what. He was running late, but I wanted to give him a quick review of my new prize. So I lined the engine up there with a string of cars behind it,...just sitting in wait. I figured we could run down to the train room in the basement, I could cranked up that loco and pull the train across the bridge, and then we could turn everything off and get underway.
According to plan the engine cranked up fine,...sound and all, BUT I advanced the throttle too fast (straight DC). The engine feel off the track sideways (to the inside of the curve) and fell hard onto the concrete floor 4 feet below !!! I was devastated
.
I did a very quick pick-me-up and put things into a box, and any little parts I saw most immediately. I was too sick to stay there and lament the situation,...might as well just go to the party, then inspect things later.
Turns out things were not as bad as I first suspected. I guess I was just lucky in the manner it hit the floor. If I recall properly it landed on its wheels.
I have often thought back about why this occurred. Here are a few of my thoughts....
1) I believe it was the inside track it was sitting on. That track is mostly 18" radius track. Even if I don't remember correctly and it was the outer track, that one is primarily 22" radius track. That's awfully small radius (lots of resistance) for the train to be running on, particularly trying to start up on,...and quickly with my jolt to the throttle.
2) There was also lots of resistance to forward motion by the fairly long set of cars still trying to climb the grade up to that turn.
3) and of course my too heavy of a hand at the throttle.
Bottom line, that 'event' is still in my mind.
But of course I am now working with much bigger radius, and much less grade, and smooth track.