How do I cange Trackmaster Percy's battery

HiI've just bought my son a set which is also incomplete (a single piece of track missing - how annoying!) Happy to sell you the whole lot for a fiver plus postage if you're interested. It's in lovely condition.
Mary

This is Bertie the bus. It stopped working so I took it apart. The Philips head (criss-cross) screw gives access to the battery. You're not supposed to take out the screws with triangular socket heads. If you do everything falls apart and you need an engineering degree to know how to put things right.
This battery holder is clear plastic, instead of black, so I can see what is happening after assembly. The red wire broke off the motor while I was looking at it so I had to resolder it. The red plate on the upper part swivels, it has an arm that holds the clutch plates together when the switch is on. The long white arm at the upper end of the gear assembly should fit into the slot of the red swivelling plate. I think it is supposed to be a noise-maker.
Bertie is supposed to go on the road, not the track and there is a plunger that cuts out the power when it is on a wooden track. This stopped working so I jumpered it to be always on.
Good luck.

Here is a video that shows how to change the battery.

THE BATTERY SHOULD GO IN THE BOTTOM OF THE TOY.YOU MAY NEED A REAL SMALL SCREWDRIVER TO OPEN IT.

There are apparently 2 different models of the Stanley train for the Trackmaster. One comes with a tar car and one is by itself. If you have the one with the tar car, the battery goes in that. If you have the one that comes solo, then you have to flip it over and in the middle of the bottom is a small screw that you have to unscrew and remove the middle wheels to take the top off. Once you have the top off, the battery is located there. I hope this helps.

My son wanted this for Christmas but we went with a couple more affordable sets. he did get others though and they dont change batteries like most of the Thomas and friends. Flip him upside down, locate all screws...find one or two that have and arrow around the screw...those are the ones you take out to put the batteries in.

My child has a similar train but it is a Thomas instead of a Percy. There is probably a screw on the bottom. My son's train has three screws and the one in the middle (+) is the one that opens the train so I can put a battery in it.

I brought one a few days ago and out of the box it talked but would not move. I returned it to the store for a replacement. I noticed that the back wheels under the on and off switch have weight on them before the train will move. If the train is not sitting on the track, it will not move, That is what makes thomas stop at the station

find it here
http://service.mattel.com/us/product_detail.asp?id=R9634&Ntt=R9634&Pn=1&Brand=Fisher-Price+Friends&Cat=Thomas+The+Train+TrackMaster

It has to be on the track to work for some reason. Turn it on, put on the track and press the button on that makes it talk.
I have to say I am pretty mad at fisher price for this one. The box says 3 AAA's included, when you open the box though you find out that they are "just for demonstration". So back to the store for batteries. Get the batteries changed, and then it doesn't work. Back to the store to exchange, and then find out it does work.