Hello! Because "was delayed" and "has been delayed" are different tenses, the structure of the sentence would be different. If you say "was delayed" which is perfect past tense, the sentence is written like this: "I was supposed to meet a friend at the train station, but unfortunately, her train "was delayed". Both parts of the sentence have to be in the past tense. "has been delayed" is the correct option because this is in the present perfect tense - in the first part of the sentence "I'm meeting a friend" is also in the present perfect tense. So the correct way to write this sentence: "I'm meeting a friend at the train station. Unfortunately, her train has been delayed". I hope that helps!

Because your sentence is in the present tense "I'm meeting a friend" you should say the train has been delayed. Has been is in the present perfect tense while was delayed is in the simple past tense.

Either one is fine.

Both sound fine to me in this particular situation. "Has been" means the situation is ongoing. "Was" means it is something that happened in the past. In this case, both can be true. The train "was" delayed some time ago. The train is still late, so you can say it "has been" delayed. EDIT: sorry, didn't mean to make this a comment instead of an answer.

You can say either.