My son's 3rd birthday was a few days ago.  My wife thought a train table would be a good gift.  She did a lot of research and found a one at Pottery Barn that she liked:

It was pretty expensive.  I can't remember exactly how much it was…I think it was around $250-$300 (plus shipping).

I looked around on the internet and found another design:

It was reasonably priced…but I thought it was the father's duty to build it. 

Below is my version.  It is very close the Pottery Barn version.  I spent about $100 in materials.  My wife bought the play mat that is on it in the picture for about $15.  The magic dimensions of all the train tables is 48" X 32".  The top is made out of MDF (Medium Density Fiber).  I highly recommend that for any table…it is very smooth and has high strength to thickness ratio.  I bought some 4" X 1" pine and nailed it around.  I added a little more height to this element.  My reason is simple:  containment.

For the legs, I just bought Parson legs from Lowe's and screwed them on. 

Very simple and sturdy.  It hard to see in the picture, but I did create the 2 drawers.  These are just a simple box of 12" X 1" pine with MDF for the bottom.  Add a few casters and it's already to go. 

Finally, I just stained it.  The whole project took about 4 hours to do.

Download: Train Table.one