http://www.state.nj.us/counties/merc...portation.html
Per the website,

"Taxi service from the Trenton-Mercer Airport to the Trenton Train Station is available for approximately $35, West Trenton Station for approximately $8. Other destinations are also available. Taxis are usually available in front of the terminal building."

If going to the airport, there are probably taxis at Trenton station but not West Trenton. However, the same page lists taxi companies that do business in Ewing Township (which includes both West Trenton station and TTN). You could probably call one or, for that matter, walk.

SEPTA serves both train stations from Philadelphia.