Every time we have rented a car from Enterprise on Olden Avenue at the Trenton train station they have picked us up brought us to Enterprise which is only 10 minutes away and we picked up our car there. When we were ready to return it we just drove to Enterprise and then they drove us to the Trenton train station. All you have to do is call them 24 hours in advance and when you arrive at the train station just give them a call and they will pick you up. They are great at Olden Avenue nicest people I have ever met.
You should call ahead. Its a fair bit of distance from Trenton transit center to Enterprise on Olden ave. If they cant pick you up, you can either take Uber or Njtransit bus.
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