If an Immovable Rod is made to stay still, and a Bag of Holding is placed around it such that the Rod is closed entirely in the nondimensional space, does the Bag hang on the Rod, or does the Rod move around with the Bag (and if so, does it remain floating motionless in the Bag)?
[RPG] Can an Immovable Rod be carried in a Bag of Holding
dnd-3.5emagic-items
Related Solutions
I believe that while the bag and rod would be locked in place, an Immovable Rod halfway into a Bag of Holding will still deactivate or be moved with enough weight or force. That includes any weight in excess of 8,000 lbs. (4 tons) or a DC 30 Strength check. Note that the effects of extreme weight and extreme strength on an Immovable Rod are different.
The text of Immovable Rod is:
This flat iron rod has a button on one end. You can use an action to press the button, which causes the rod to become magically fixed in place. Until you or another creature uses an action to push the button again, the rod doesn't move, even if it is defying gravity. The rod can hold up to 8,000 pounds of weight. More weight causes the rod to deactivate and fall. A creature can use an action to make a DC30 Strength check, moving the fixed rod up to 10 feet on a success.
If more than 8,000 lbs is placed on the Immovable Rod, even if it's placed halfway in a Bag of Holding, it deactivates and the rod and bag fall. A strength check could have multiple results. A creature can pull the rod from the bag (causing the bag to fall off), push it further into the bag (leaving it in the bag's pocket dimension, meaning the bag will fall), or push the rod and the bag 10 feet (the rod stays halfway in the bag). Also consider that weight put on the button could cause it to shut off immediately, even if it's less than 8,000 lbs.
Up to that point, however, I would rule that any force placed on the rod is no different with or without the Bag of Holding. Even placing a ton of weight (2,000 lbs) on the rod shouldn't puncture or break the bag, as the rod is fixed in space and supports the weight entirely. However, I wouldn't say there's any benefit in terms of stability gained by attaching a Bag of Holding to one end, and neither does the linked answer. It simply says that attempting to move an active Immovable Rod into a dimensional space in a Bag of Holding would require a strength check to do, as it's considered to be moving the rod.
Keep in mind that the linked answer is not written rule, merely an interpretation of the effects of an Immovable Rod, the pocket dimensional space of a Bag of Holding, and the nature of physics in the D&D system. It does not override the rules of an Immovable Rod, unless the DM decrees it so.
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.
Best Answer
Nice theoretical question. :) And I partially disagree with the other two (?) answers (that I've seen so far.)
I'd say you could place the Bag of Holding around the Immovable Rod, or push the Rod into the Bag, though with a very, very serious effort -- a DC 30 Strength check -- only. Why? Because
"the rod does not move from where it is", ie. it has a pretty fixed position in space, and
"The bag of holding opens into a nondimensional space", that is, the inside of the bag is a different place,
...and the rod does not move from where it is, unless you apply serious force (a DC30 STR check, as per the rules.) Pulling the bag over the rod or pushing the Rod into the Bag would cause the Rod to move, from the original dimension to the bag's dimension. It would be like trying to touch two similar magnetic poles to each other - which can be done with great enough force. So, you could pull the Bag over the Rod, etc... with a DC30 STR check.
If you activate the Rod within the bag, its position will get fixed within the Bag's "nondimensional" space (whatever that means) only, so you'll be able to move the Bag anywhere on the outside, with the Rod in a fixed position in it.
In case you activate the Rod when it's only partially covered by the Bag, moving the Bag off the Rod, or moving the Rod along with the Bag would be a DC30 STR check, since the Rod would have a fixed position both within and without the bag. (Well, depending on your view of physics and magic, you could lower the DC in such a case to your heart's content.)