Is the comma correct in the sentence?

I would like you to help me to explain the usage of the comma before "was" in this famous quote from Grand Theft Auto: San Andreas. I’ve already looked through many grammar textbooks but haven’t found the exact answer to my question.

All we had to do, was follow the damn train, CJ!

In my estimation, there seems to be either just a common mistake or author’s solution to emphasise this part of the sentence.