And how is what the kobold knows determined?
The DM decides it. Either by their perrogative via world-building and NPC control or by random chance, their choice.
Would the players have complained if the DM judged the kobold DID know everything and spilled his guts??? NO! They would have been thrilled. Yah! Woo-hoo!!! We captured a kobold and got the info we wanted. Yeah, us!!!
It is only when the DM decided
against the players that they felt it was railroading when he provided no useful intel.
If the DM just spontaneously decides it knows nothjng useful, that seems like poor DMing to me. Call it railroading or not, it seems crappy.
How is it "crappy"? It is just something that doesn't go the players' way. Unfortunate for the players yes, but nothing wrong with it as a DM.
If the DM is going to make every attempt to justify this… like your (i) to (iii) above… then hasn’t the DM already decided the outcome?
Yes, the DM has. Just like when the PCs get to the door and the DM decides it is locked. The DM shouldn't have to "roll randomly to determine if a door is locked or not". DMs decide how the world operates
all the time. It is part of their job as world-builder and storyteller.
NO DM in ANY GAME determines
everything by random roll that I know of. And if such a game exists, don't tell me about it, I'm better of not knowing about that crappy of a game. After all, the players don't decide everything their characters do or know by random roll, why should the DM?
I mean, instead of interrogating a prisoner, imagine the PCs were attacking a combatant. If the GM gets to just decide how it goes… no application of to hit rolls or ACs and so on… that would seem like poor GMing, no?
Ok, good example! The DM should absolutely NOT just "decide if the interrogation will get the kobold to spill the beans".
The kobold breaking under the pressure of interrogation is the kobold's AC, it's "resistance" to the "interrogation attack".
This could be a static value, a "DC" the players have to get, or could work in the other direction and be a morale check the DM rolls for the kobold against a set morale value.
The kobold knowing information is the kobold's HP. In this case, knowing nothing would mean the kobold has like 1 hp.
So, the DM does get to set the amount, if any, of the information that kobold might know--just like the DM sets the AC for the creature the PC attacks
Attack rolls have very clear success and failure results. If I don’t roll equal or higher than a creature’s AC, then I miss.
So why not apply similar rules to an interrogation?
Sorry, see above.
The action in question is interrogating the kobold. They learned nothing… the interrogation failed. No rules were used to determine the outcome… it was just the DM deciding in the moment. Or perhaps deciding weeks before and actually writing down “if the PCs try to capture and interrogate any of the kobolds, they don’t learn anything of use!”
It’s a railroad.
As long as the DM used a mechanic to determine IF the kobold would succumb to the interrogation (morale check, reaction roll, etc.) the interrogation, itself, did not fail. That they learned nothing useful is not the same thing.
The DM decides what NPCs know or don't know. It is really that simple and not railroading. Now, if the DM didn't use any system to determine IF the kobold would give in?--that would be railroading.
If that’s what the GM wants, why introduce the chase and the choices for the players? Why go to all that trouble? If the fight MUST happen… if the DM is so set on it that every but of judgment they apply to the game pushes thjngs toward the fight… why go through the motions? Why bother letting the players think the fight can be avoided?
This depends entirely on what the situation
actually is that the DM is setting up for the encounter.
1) The fight can be avoided. The DM sets up the scene of the chase and the PCs have a chance to get through the door before the fight begins.
2) The fight cannot be avoided (it's the encounter!). The DM sets up the scene of the chase and the PCs get to the door but it is locked. Roll initiative.
There are way too many other factors and variables to know what precisely the set-up is.
But then the fight is already happening, so picking the lock at that point doesn’t seem to hold the same appeal.
Well, it doesn't hold the appeal to avoid the fight altogther, certainly, but it can hold appeal as a means of escape from the fight in progress or as a tactical bottle neck during the fight once opened.
If the DM decides the situation… the layout of the city street, the presence of the alley, the presence of the door, whether it’s locked, how far the pursuers are, and when they will catch up… and can’t find some place in all of that for something more than “DM decides” then I’d say they have a lot to learn as a DM.
LOL you do you, man.

That is all totally good scene development IMO and very much D&D!
What would YOU DO differently, huh? Where in that chain of decisions does the "good DM" do things differently?
- You don't design the layout? Well, I think you probably do unless you roll the city layout randomly?
- No alley? Ok... then where do the PCs go when they encounter the killers? Wherever that is, you just decided that instead of the alley.
- No door? Fine, then the alley is a dead-end and there's going to be a fight. Which is why it's an encounter.
- Not locked? Fine, then no encounter--the PCs escape. You just narrated the choices for where the PCs can go and they decided where to go. But then the "encounter" was never going to happen unless the PCs just decided they wanted to fight--but given the original synopsis fleeing seemed to be the decision. But then this begs the question: Why THAT door? Where does it go? Didn't you just "railroad" the PCs into going through the door--just like YOU wanted???

- How far the pursuers are? Close enough to begin the encounter, otherwise why bother??? The DM could randomly roll this (I often do), but the range of the roll is still determined by the DM, right? So, close enough to begin the chase, but far enough that it isn't an immediate fight.
- When they will catch up? Predetermined by the encounter distance and the speed. No DM decision here other than the above distance factor.
Given all that, you tell me what you would do differently and how that makes for a better DM, because according to you, I "have a lot to learn as a DM". Teach, man, teach!
That is not the example that I posted:
I did not talk about the locked door as a challenge.
The example I gave had the GM telling the players that the door is locked, and that while their PCs are trying to open it, the pursuing killers catch up.
Do you agree that that is bad GMing, and railroading?
Ok, just to be clear then since I might have misunderstood your scenario (and I want to make certain we're discussing the same thing of course!):
What is the scene the DM is setting up? The chase and possible escape OR the encounter with the killers in the alley?
If the first and the DM just narrates everything up to the fight, that is railroading as I see it. The DM gave the players no choice on where to go (you go into the alley) or chance to open the door before the fight begins (the killers engage you while you are trying to open the door). Ultimately, all this narrative choice simply led to the second option...
With the second, the scene is the fight in the alley. That is the challenge to the players. The presence of the locked door is part of that challenge. You might see this as railroading, I see this as setting the scene for the encounter.
I'll give you a scenrio in my own game we were in the middle of when we had to end our last sesssion. The PCs are traveling by boat back to town after completing their last adventure.
SCENE: While navigating the swampy waters of the wide, swollen river, the PCs are surrounded by forests on the far banks. Flying overhead are 3 manticores. With the people on the boat not being quite at all, the manticores were automatically aware of them (they are out hunting after all--there is no roll required here). One of the PCs and one of the crew spot (successulf perception) the manticores flying far above and give the alarm. The scene is set, battlemap ready, and the battle ensues. Roll initiative.
Is that railroading or scene setting?
FWIW, I randomly rolled up 4 encounters during the six-day journey. This is the 3rd encounter on day 5. The fact of the encounter is rolled, the difficulty and CR ranges is rolled; I picked the manticores, and according to the rules in the DMG, built the encounter to Deadly given the rolled difficulty. Even the time of day and weather of the day is rolled randomly. Does knowing all of this change you answer if you felt I was railroading the PCs into the battle?
Purely due to an arbitrary decision by the GM, to make the Kobold incapable of answering questions, so that we - the players - couldn't pursue our plan of gathering and acting on intelligence.
And would you have been upset if the "arbitrary decision by the GM" was to give you the information you expected and made you happy that your interrogation was "successful". Fun, fun, fun, after all???
The action that failed was the attempted interrogation.
No, you succeeded (I assume?) in breaking the kobold, but the kobold didn't know anything useful.
If the GM just makes up arbitrary fiction to make actions fail, I consider that railroading.
Really, because they do that all the time in the party's favor and no one see it as railroading then...
This is no different from knowing - which I and my fellow players did know- that we have no agency to control our characters as we wish, since the GM can just declare failures again, and again, and again, making up whatever fiction they want that will explain why our PCs do not succeed at anything they attempt.
You had total agency and control over your characters, what they did, etc. What you lack control over, as players, are the things the DM controls---which is
everything not your characters.
The DM didn't strike the party dead with a lightning bolt from the sky to end the interrogation, killing off the PCs. You still had agency and control over your PCs. How did they react to the kobold's lack of intel? Did they kill the kobold? Use him as bait to capture other kobolds? Bribe him to join their sides, helping them lure other kobolds into traps?
You, as a player, control your character--that's it. The DM can use systems (if present) to determine outcomes, or choose them according to the narrative of the game.