Can “I'm waiting for the train” use auf or für interchangeably?

Ich warte auf den Zug.

Meaning, “I'm waiting for the train”.

“für” always means “for”.

“für” forces the noun to be accusative.

“She is baking cookies for the family.”

"Sie backt Kekse für die Familie." 

“auf” is a presposition that can mean on, to, in, at, onto, upon, toward.

So using “auf” would make the English phrase say, 

"I'm waiting on the train [to arrive]”. WHich is the correct meaning.

I keep thinking “für” but it's “auf”. Is it also correct to say this too?

Ich warte auf für Zug.

Hallo James1!

The correct way of saying that is “Ich warte auf den Zug” as you have already identified. Unfortunately auf and für cannot be used interchangeably here. If you use für here, it changes the meaning. Take for example,

Ich warte auf Emily. - I am waiting for Emily.  

Ich warte für Emily. - I am waiting for Emily. 

In the first sentence, you're waiting for Emily to arrive while the secon one implies that you're waiting for something but you're doing it for her. 

Hope that helps!

Viele Grüße, 

Julia