The commas in your example are bracketing commas used to set off the bolded interruptions. Bracketing commas can come in pairs in the middle of sentences or by themselves at the start or end of sentences. They serve to add more information to complete sentences.

After two hours, the train came to the station.

Before I could talk to her, she was gone.

According to Trask himself, the commas in your examples are bracketing commas used to separate the bolded interruptions. Bracketing commas can appear as a pair to enclose an interruption in the middle of a sentence. They can also appear alone to separate an interruption at the start or end of a sentence. They serve to add more information to sentences that are complete.

After two hours, the train came to the station.

Before I could talk to her, she was gone.

The commas in your example are bracketing commas used to set off the bolded interruptions. Bracketing commas can come in pairs in the middle of sentences or by themselves at the start or end of sentences. They serve to add more information to complete sentences.

After two hours, the train came to the station.

Before I could talk to her, she was gone.

The commas in your example are bracketing commas used to set off the bolded interruptions. Bracketing commas can come in pairs in the middle of sentences or by themselves at the start or end of sentences. They serve to add more information to complete sentences.

After two hours, the train came to the station.

Before I could talk to her, she was gone.