I won't try and dissuade you from the train - I did a similar train ride from Sapporo to Tokyo, stopping in Aomori and Sendai along the way. If you have any engineer-nerdness in you, you'll pass through one of the world's engineering marvels along the way, the Seikan tunnel. It's not very interesting to look at from the train, but I found it neat anyway.
I'm not sure if the regular game atmosphere is held over to the preseason games. If you're an american baseball fan, you're in for a treat at Japanese baseball games. It's a very different experience - more like a US College Football game than a MLB game, to me.
If you can, try to get seats in the home team's cheering section - these will be outfield bleacher seats, but with the most intense fans and best experience. My experience is that the fans in these areas are happy to absorb foreigners into their midst if we show even a hint of trying to fit in.
If you don't sit in that section, be aware that the side of the stadium you sit on will determine the mix of fans around you. Home team fans will sit on the side of the stadium with the home team dugout and vice versa.
If you're willing to shell out a few yen for team-specific paraphernalia, it will help break the ice with your nearby fans. If things go well, you can get them to teach you the player-specific cheers as you go along.
I've only been to games hosted by Hanshin Tigers, Yokohama Baystars, and Yakult Swallows - I'm envious of your trip!