Por + infinitive; Para + infinitive.

[fglorca]

Por + infinitive:
The emphasis is on the fact that these actions should have taken place by now, but for some reason, they haven’t:
El tren está por salir. The train has yet to leave.
La cena está por cocinar. We still have to cook dinner.
Estamos por terminar. We still have to finish.


Para + infinitive:
However, with these three, the emphasis is on the fact that we only have a short amount of time left before any of these actions occur:
El tren está para salir. The train is about to leave.
La cena está para cocinar. The dinner is about to be made.
Estamos para terminar. We are about to finish.


Would this interpretation be accurate?

Many thanks in advance.

[Tomisimo]

Very good question. Here's another one and how I would have translated each version, but I'm curious to see if I'm off on this one.

Estoy por salir. = I'm about to leave.
Estoy para salir. = I'm ready to leave. (Or I'm willing to leave?)

[AngelicaDeAlquezar]

As far as I can tell, it's only regional use. In Spain it's more common to say "estar para" and in Mexico (and probably most of Latin America too) we rather say "estar por", when we mean we are about to do something.

- El tren está por/para salir. -> The train is about to leave.
- La cena está por/para cocinarse.* -> The dinner is about to be cooked.
*Notice that a person can cook, but the thing can only be cooked.
- Estamos por/para terminar. -> We are about to finish.
- Estoy por/para salir. -> I'm about to go out.


When something or someone isn't quite ready yet, we tend to use "todavía"/"aún":
- El tren todavía no sale. -> The train is yet to leave.
- Todavía no está la cena. -> Dinner is not cooked yet.
- Aún no terminamos. -> We still haven't finished.
- Todavía no estoy lista para salir. -> I'm not ready to go out yet.


"Estar para", at least in Mexico, can be used to express that we're ready or in disposition to do something.
- ¿Qué tal estás para una tarde de dominó? -> How would you like a domino afternoon?
- No está como para comer afuera. Estamos en cuarentena. -> It's not approppriate to eat out. We're quarantined.
- Ana quería contarme de su pleito con Juan, pero para chismes estoy yo. -> Ana wanted to tell me about her fight with Juan, but I'm not in the mood for gossip.