A man stepped onto the overnight train and told the conductor, “I need you to wake me up in Philadelphia. I'm a deep sleeper and can be angry when I get up, but no matter what, I want you to help me make that stop. Here's $100 to make sure".

The conductor agreed. The man fell asleep, and when he awoke he heard the announcement that the train was approaching New York, which meant they had passed Philadelphia a long time ago. Furious, he ran to the conductor. “I gave you $100 to make sure I got off in Philadelphia, you idiot!" “Wow," another passenger said to his traveling companion. “Is that guy mad!" “Yeah," his companion replied. “But not half as mad as that guy they forced off the train in Philadelphia."

(English2Go, No 7,The Reader's Digest Association, 2005. P. 80.)

In “ ...the train was approaching New York" a gerund is used as a/an