My two sons (22 months and 3 1/2 - 19 months apart) love, love LOVE anything with trains. My older son got his first Thomas train at about 1, but wasn't too interested until about two. The second one has been interested since he was about 1, mostly because his older brother plays with them all the time.
As I am typing, they have woken up and ran downstairs to play with their trains again like they do every day. It has provided them months and years of entertainment at this point.
You can actually get some of the wooden trains on Learning curve's Web site http://shop.learningcurve.com/thomaswoodenrailway/categor... for $2 or so - they are usually $10- $20 each in the stores.
There are 3 types of Thomas track/play sets. Wooden, take and carry and another one so just make sure you get the ones that work together if you do.
Best of luck!