Whenever you see a car that offers Bux or Train Pieces, deliver them as fast as possible. The value that you get from exchanging Bux carts for coins at the bank will usually exceed any value you can get from regular coin carts. The Train Pieces are needed for building new trains (engines) or fuel carts for trains. Although you will want to have as many trains running as possible at first, having no fuel in the tanks will require longer waits and cause a loss in revenue. After you have five trains running, build fuel cars for them to keep them running longer.