Kmart offers the Caterpillar Toys Iron Diesel Train Set for $16.89. Plus, Shop Your Way Max members bag free shipping. (Not a member? It's free to sign up for a 90-day trial; otherwise, shipping adds $6.99. In-store pickup is also available.) That's the lowest total price we could find by $7. The set includes seven feet of interchangeable track and four vehicles. It requires two AA batteries, not included.
Update: It's now available for in-store pickup only.