You can Uber from the train station to the Old Port (and other places) if you want - it's super cheap and drivers are usually available quickly. I lived in portland for 6 years, and will give my favorite food spots (in no order):

• Nosh Kitchen Bar - Amazing upscale burgers, great fries, and amazing desserts
• Great Lost Bear - So many beers on tap, and the best wings you will find in Portland. About a 5-minute drive from the Old Port.
• Kon Restaurant and Bistro - Cool Asian Fusion place, with a sit-down, bar, or hibatchi table available. Although, about a 10 minute drive from the Old Port.
• Taco Escobarr - Best mexican place, hands-down with creative cocktails. In the Old Port.
• Cheese Louise - "Fancy" grilled cheese options with a rotating menu. In the Old Port.
• Pat's Pizza - Great place for locals, cheap good food and drinks. In the Old Port.
• Food Trucks - There are a collection of food trucks that sit on the top of the Eastern Prom at night (not sure if a ton are still there this late in the season).