DUPLO Train Shopping Resources
You might consider buying duplicates of some of the train sets to get more tracks and bridges, or you can buy the pieces individually on BrickLink, the largest online LEGO marketplace. For example if I pull up the #10508 set on BrickLink and look at the parts selection, I can see that the straight train tracks are available for about a dollar each from many different sellers. Same for the curved tracks. Here are some direct links:

Straight tracks: http://alpha.bricklink.com/pages/clone/catalogitem.page?P=6377&amp;idColor=85#T=P&amp;C=85

Curved tracks: http://alpha.bricklink.com/pages/clone/catalogitem.page?P=6378&amp;idColor=85#T=P&amp;C=85

And you can do the same for other pieces, including the bridge section, etc. Here is the page for the main set where you can look at the parts selection and see the ones you would like to get more of: http://alpha.bricklink.com/pages/clone/catalogitem.page?S=10508-1#T=I

And here are all the DUPLO train sets ever made: http://www.bricklink.com/catalogList.asp?catType=S&amp;catString=167.441