We will be arriving Rome by train. What is the best location in Rome for restaurants and accommodations?

Near Piazza Navona or Campo dei Fiori. Lots of places advertise that they are near the Spanish Steps, but I don't think that's very central. I also don't think the Vatican area is very central.

Thank you. Is this area easily accessible from the train station?

With luggage, I'd take a cab. Central Rome is small, so cab fares are not expensive.

One reason to stay near the Piazza di Spagna is that it is three stops from the railroad station by Metro and Piazza del Poopolo and Piazza Navona are both walkable.

We always with stay within five or six blocks of the train station. Primarily for easy of access from the train station and it is a more modestly prices. Lots of nice 3 star hotels in that area.

I too recommend staying near Piazza Navona and Campo di Fiori. They are not really convenient to the metro, but in all honesty Rome's metro system is not the greatest. Its super crowded and not very extensive. However, the historic center is fairly small and very walkable, and taxis from Termini (the main train station) to Piazza Navona are relatively cheap.

Tracy

There are a lot of people on these boards who will swear that the BEST location for restaurants is Trastevere - and I've eaten well for little money in Testaccio, right nearby. Fewer hotels in Trastevere, but also many b&bs and apartments for vacation rental.
Depends on what you want - "best" means different things to different people, and if for you it means "many sights right outside my door", that's one thing. If it means "a little to the side of the center, where the tourist/resident ratio is a little more Roman and less visitor, that's something else.

It surely is NOT, acc. to me, the tourist areas of Piazza Navona-I like Monti-the area around the Colisseum and Via dei Serpenti, but also, maybe more, the area around Esquilino, and the Santa Maria Maggiore Basilica-Via Labicana, and Via Cavour-close to everything, with ALL transportation, hop on/off buses, internet points, great little local restaurants within a few minutes walk.