At age 4, our son was a huge train fan..so we created his Train party... For the invitations I found a free internet clip art of switching tracks, and added "Follow the Tracks to Port's 4th birthday!" I made black engines out of oak tag which we used throughout the neighborhood to direct guests. I also made a large station sign "Port's Station" mounted above the door and a RR crossing sign at the entrance.  The floor was the best part.. We have wood floors so I created RR tracks using blue painters tape in the main room, with a switching track heading into the dining room.  Our son was so surprised.. he really loved it all ! The balloons were Thomas the Train colors, and I created a "Thomas" cake. Included in the party favor bag was a copy of Port's favorite music cd.