"I purchased this set on the 13th September. When I finished the build I was very happy with the set. The only modifications I've done to the locomotive were changing the color of the boiler, I did this with spare bricks and this did not have any affect on how the train ran.
I've been running this set round my Lego city, and I haven't had issues until recently. I noticed the train struggles going round corners; the train won't even move unless the speed "level" is on 3 or over (using my phone as a controller) I tested this with and without the cars. Testing the tender itself, you can hear and see it slowing down while going round the corner (I have not modified the tender in any way).
I've looked at reviews, forums and videos online and this issue hasn't been raised.
I've tested 3 different types of triple A batteries, and I think there could be a problem with the motor itself.