I recently bought the 10508 train set for my 3 year old and he just loves playing with it. I am aware of the 10507 and 10506 sets to expand further.
Are there any other resources available to buy locomotives / coaches and track pieces, bridges, etc. that are not part of the above 2 mentioned sets but will work with the 10508 set?
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&idColor=85#T=P&C=85
Curved tracks: http://alpha.bricklink.com/pages/clone/catalogitem.page?P=6378&idColor=85#T=P&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&catString=167.441
