I was reading a book called "The Penguin Guide to Punctuation" by R. L. Trask. It seems that the book doesn't explain all the uses of comma. It says "There are four uses of the
comma are called the listing comma, the joining comma, the gapping comma and bracketing commas".

In the following example below which type of comma is being used?

- After two hours, the train came to the station.
- Before I could talk to her, she was gone.

Apparently, the above usage of commas doesn't meet any comma rules described in the book.