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I ended up buying Duracell batteries and they work fine in the stock battery box. I don't like to drill in Lego much . Does anyone know what runtime I should expect with 800 mAh rechargeble batteries? I get about 2 hours of continuous run-time when the train is at half speed.

I'm not sure what to expect as far as run time is concerned. This is my first PF train. The non-rechargeable batteries I tried drained really fast. I got about 30 minutes run time at top speed from the pre-charged Duracell nimh batteries. I am charging them now for the first time and yet have to see what run time I get. I will try a new motor as I want to buy a spare motor anyway. I will look into the O-rings. Thanks for mentioning that.

That is good to hear the PF trains run well on 9v tracks as well. I found out now the train has much less wheel spin and also runs faster when running backwards so when the motor is pushing instead of pulling. I will see if I can add some additional bricks inside the engine. So what I will try to do additionally: - replace the batteries by Duracell nimh which fit the battery box as mentioned above. (Apparently, they don't sell Rayovac or Energiser batteries over here at online stores) - replace the motor to hopefully prevent the batteries from draining too fast - replace the battery box by the one Lego has shipped to me if the above fails Thanks for your replies so far. I will let you know how it worked out in time. Finally, I have replaced the batteries with rechargeable Duracell batteries and the train runs fine now although still has some wheel spin. Running it backwards gets rid of most of the wheel spin.

Thanks for your reply, I have checked this and it appears to be the case indeed. My rechargeable batteries have larger diameter tips than the non-rechargeable batteries. I will try to modify the battery box or try again with another brand of rechargeable batteries. What about the continuous wheel spin, did someone experience that? Or should I be running the power function trains on plastic tracks for better traction rather than the old 9v metal tracks?

Hi all, I own some push-along trains, 4.5v trains, 12v trains and 9v trains and recently extended my collection with the 7938 power functions train. Unfortunately, I have had nothing but trouble with it so far. Things I ran into so far have been: - Unlike the manual claims, the power functions battery box does not work with rechargeable batteries, only with non-rechargeable batteries. The green light does not burn when switching on the battery box with rechargable batteries inside. I am using the same rechargeable batteries in other devices and they work fine there. So broken batteries are not the problem. - The (new alkaline) non-rechargeable batteries are drained within 10 minutes of running. The motor does get reasonably warm during that time but not exactly hot. Which means I'd have to spend a fortune on batteries to keep it running. - There is a large amount of wheel-spin all the time and the train makes a lot of noise when that happens. It just doesn't run as good as the old 9v system. I suspect both my battery box and motor are broken. The train is brand new and I have a lot of experience building Lego trains since decades and I am sure I have build the train correctly. I am not considering to buy the Lego rechargeable battery box 8878 as it is too expensive. I am running the train on the old style metal 9v tracks. What I have tried so far is call the Lego service line and explain the issues. They are very kind and helpful and will ship a new battery box. I doubt however if that would solve all the issues. Has anybody ran into these problems before? And any thoughts of what might be the issue?