8 hour layover

We have an 8 hour layover in Tokyo. Where could we go by train in that amount of time?

I assume you arrive at and depart from Tokyo's Narita Airport.

Considering the fact that it takes 1 hour to reach Tokyo from the airport, I don't think it is worth the effort, because you would end up with only about 2 hours of time in central Tokyo. Instead I would recommend to see some of the attractions around the airport.

There is a nice temple:
http://www.japan-guide.com/e/e6401.html

A large AEON shopping mall and a nice open air museum (Boso no mura) nearby.

I saw on the news recently that they are currently trialling a special Narita city bus tour for that kind of stopover which takes you to the shrine and other areas of interest in Narita city. Maybe you could ask about it at the airport information desk.

don't forget that you will have to go through customs and immigration-count on 1 hr-. then need to be back at the aiport 2 hrs before departure. This roughly leaves you 4 hrs free time in Narita city.

When I have 4 hours between flights, I just go into Narita City (1 stop on either train) and wander around the temple area.

If I have 8, I head a couple of more stops to Sakura, and check out one of the better museums in Japan, the National Museum of Japanese History.
http://www.rekihaku.ac.jp/english/index.html

It closes at 5 PM so it depends when your 8 free hours are.

Check out this and plan what you can do.
http://www.chiba-tour.jp/narita/sight/sight_5.html