I have another Thomas here to convert to battery R/C. Before I do the conversion I wanted to show readers how easy it is to do a simple modification to Thomas so he can run on Track Power or Trail Car battery power. Following are three pics of how I would wire Thomas. Take care removing the body from the chassis. Lift the rear up and push forward to disengage the movable eye levers. Then I drilled a suitable hole in the floor of the chassis. Simply reach under the floor to switch from one to the other. The first pic shows how I unsoldered the two sets of pick up wires from the little pcb on the end of the motor. Then I kept them joined together and added an extension for each colour so they could reach the switch. The joins were covered in heat shrink tube. The thicker black parts. The extensions go to one end of the switch.
Next I added two new wires from the little pcb on the end of the motor to be connected to the centre terminals of the switch.
This pic shows how I drilled a hole in the rear crossbeam to allow a two wire connector cable to be fed through and up into the loco. These two wires were connected to the other end of the switch.
Tuck the wires out of the way and refit the body. To make life simple replacing the little levers for the eyes in the slots on the actuating mechanism, I removed the face and taped the eyes in the middle. Then when the body is back on it is much easier to get the levers in the slots. I hope that helps. I will post the battery R/C + MyLocosound conversion later this week.