This fix is from a few years back.
One day, a train arrived outside
One day my wife turned up with a large bag of train track and asked me to pop out to the car to get the rest. I also found a small ride on train in the boot of the car. "I picked it up from the charity shop, it's not working" she said "but I'm sure you can fix it".
I looked the train and soon discovered it had a 6v Lead Acid Battery under the seat. I used a multimeter and determined that it was quite flat.
I did not have a charger so did some googling and discovered that it was possible to make one. I found an L200 regulator kit in the bottom of a drawer and added a high wattage "sense" resistor to limit the current. I also added a couple of bits of old PC heatsink, one to the regulator and one to the resistor and boxed it all up. This allowed me to charge the battery.
Once I had a charged battery I tried powering it up but nothing happened, so I needed to take it apart. To get to the electronics I pretty much had to strip it right down. I started from the bottom where I found some screws and followed the wires and screws till I had exposed all of the circuitry. I was quite an involved process so I made notes as I went. These were written up as an iFixIt guide.
Working through from the battery I used a multimeter to see where the power was getting to. My first stop was the main power switch which is togged by a lever on the top of the train. This is a dual throw switch but only one pole is used. The connected side was not working so I checked the other. I was luck in that this was working. So I swapped them over and reversed the switch by 180°, I could now get power to the motors and some lights and sound but the train only went backwards.
Next up was the lever on console that controlled the direction of the motors. This was a simple mechanical problem the lever had been moved too far and was no longer activating the switch. A simple repositioning resolved that problem.
Also one of the sound switches was not working. At the time I had no suitable replacements so I gave it a clean with some contact cleaner. It was tempremental but sometimes worked to generate a sound effect.
My last fix was cosmetic but I think makes a good improvement. The bumpers were paper stickers and had deteriated. I used some metal from some old computer parts and plastic from a shreader to make some new ones. These were turned on the lathe and glued in place with some builders adhesive.
Over the next couple of years the train was used on sunny days but we noticed that both the train was struggling to drive along and my daughter was stuggling to fit on it. So we bundled it all up and returned it to the charity shop where we'd first bought it from. Hopefully at least one other family will get the same enjoyment from it as we did.