The Red Bandits have boarded the train, hidden among the other passengers. They wait an hour or two – to be sure the train is far from any city – then, on the boss's signal, they all pull out their weapons. The boss shouts, "Everybody down! This is a robbery!"
As they thieves rush from car to car, robbing each passenger of jewelry and loose coin, our adventurers realize they need to do something or lose their recently-won loot. The rogue pulls out an Immovable Rod, places it against the car's door to bar it shut, and presses the button.
What happens?
Does the Rod use the ground (or the Prime Material Plane) as its reference frame for "unmoving"? Does the Rod stay still relative to the train car? Does it stay in motion in a straight line, which works fine (albeit with some shaking) until the train tracks turn?
Obviously this ultimately falls to DM discretion, but is there any rule or description that leans toward a particular answer? (I've tagged it for 5e, but I'll take an answer from any edition.)
Best Answer
Per the rules, I think the Rod isn't moving and the train is. The rod flies "backward" through the train, punching holes in doors and walls and people. Bad news all round.
But that's a lame outcome. The party is trying to find creative solutions to problems, and as the DM, we want to reward them for doing that. Tell the rogue: "Ordinarily, the rod would align to the reference frame of the ground. It sounds like you're trying to align it to the reference frame of the train floor instead. What skill are you using to do that?"
You wrote that you'd take an answer from any edition, so I'll suggest that this is a great ad-hoc use of the Use Magic Device skill from 3.5e or Pathfinder. On a good roll, the rogue figures out how the rod is identifying its frame of reference, and manages to trick it into using the train floor as its frame of reference instead of the ground. On a bad roll, the Rod goes out of control as before, but the party at least understands why their plan didn't work.
If the group is playing 4e or 5e, there is no Use Magic Device skill, so you'd probably have to call for an Arcana check instead.