Brio, Adventure Tunnel 33481
If you want to incorporate some figures into your train set then this will add a nice element of excitement to your child's play. There's a small footbridge that passes over the top of the tunnel and whenever a train passes through it a magnetic sensor inside sets off one of four different sounds (you can activate them by pressing the button on top of the tunnel as well). Suitable for children aged 3 years and up. Requires 2AA batteries (not included).