I don’t own one, but the loco may not be the issue in and of itself. First consider that these are all hobby items not intended to be operated over extended periods of time. Most businesses that run trains in this fashion rarely do any form of maintenance on them until they quit running. So let me run down a list of things I have found in doing investigations exactly like you are doing.
First, look the locomotive over carefully. inspect the drivers and pony/trailing truck wheels for excessive wear both on the tread and the flanges. Once the flanges start to wear, the back to back on the wheels may be OK, but as the flanges wear the gauge of the wheels gets wider.
Looking closely at the track picture you provided, the track appears to be Bachmann stamped steel track. From the photo I don’t see any wear or abnormalities that would cause a derailment, but consider that is only one picture it is certainly not an overview of the total picture. Also consider that conditions before the curve in question could be contributing to the derailments as well - ie, an ‘S’ curve of 4’ diameter track can cause a lot of rollingresistance which could be putting excessive side load on the rear of the engine/tender.
Inspect the trucks on all the cars as well. Have they been kept lubricated, if not this will add to the rolling resistance in general. Be sure the journal boxes on the trucks do not have excessive wear, a steel axle will wear a plastic journal box even when well lubricated. Do the cars have plastic or steel wheels? Plastic wheels will also increase rolling resistance.
Without a personal inspection where one discovery leads to another not necessarily anticipated, I don’t want to go any further into speculation that may be fruitless. Visually watching the loco derail, and at speed, can sometimes tell a lot. Then slow the train some and try again. Keep watching, you will likely observe the cause. I hope this has given you some direction to start a comprehensive inspection and hopefully a solution.