The most obvious way to encourage train-use is to take the train yourself whenever possible. If you sometimes travel relatively short-distances by plane, maybe you could convert those trips to train rides? If you live in a suburb, maybe there are light-trains that go into the city.
You could also write to your elected representatives (on various levels - city/state or province/federal) and ask for a better train infrastructure, suggesting that it would make their city/state or province/country more competitive, slow down road wear and accidents, help clean up the air, etc. That's where joining an organization and bunching up with many people can pay off. You could even try to make it a local electoral issue by writing to local newspapers before an election. But as with any political action, it can be very hard to get the attention of the right people, and since you probably aren't a lobbyist with lots of money to throw around, things will probably be frustrating. Our best bet right now is that rising oil prices will make the transportation of goods by truck more expensive and that trains will gain back some marketshare, leading to investments in the infrastructure and, indirectly, better service for commuters and travellers too.
Rising oil prices could also lead to the use of more biodiesel fuel. In that area, your options are also to write to railroad companies and suggest it, or to buy some of their stock and go to a shareholder meeting so that you can suggest it as a shareholder.