This post is intended to offer some assistance to newcomers to the O Gauge hobby wishing to expand a Lion Chief set with Accessories and explain why certain Lionel Accessories may cause Locomotives to stall and offer some suggestions for how to resolve these issues. The information in this Topic is for a Locomotive set that works correctly without any accessories attached. If yours isn't, then there are many other topics on this forum to help with that.
If you're using the DC Power wall transformer that came with a Lion Chief set, it may not have enough electrical power to provide the wattage [Volts times Amps (current)] required to simultaneously run a Train and accessories with higher electrical current demands. Some of these accessories include the 6-12020 Uncoupling Track, 6-12054 Operating Track (using the uncoupler), and the 6-62162 Auto Crossing Gate (image below). Lighted passenger cars also draw additional power. This list of accessories is not all inclusive, but these are some of the most commonly added ones when first expanding a Lion Chief starter set.
The transformers included with many LC (Lion Chief) sets is 18Volts DC, either 36 or 54 Watts (images below). Lionel recommends operating most of its accessories with AC voltage, even though they might sometimes work ok on DC. Check the accessory's manual to make certain and also note their recommended voltage might be less than 18VAC. Most Owner's Manuals can be found on Lionel's support site at https://www.lionelsupport.com/service-documents/ Presently, Lion Chief Locomotives are the only new Lionel locos designed to work on either 18Volts AC or DC. Most other stock locomotives are designed to only work on AC.
Uncoupler and Operating Track - If your locomotive is running as expected but then suddenly stalls when the Uncoupler is activated AND if the Uncoupler works correctly without the Locomotive running, then you probably need additional electrical power in your layout.
Crossing Gate - If the answer to ALL 3 of the next questions is yes, then you probably need additional electrical power in your layout.
1. Is your locomotive running as expected, but then suddenly stalls when the Gate is triggered?
2. If you remove either one of the wires (doesn't matter which) from the crossing gate, does the Locomotive run all the way around the track without issues?
3. Reattach the wire to the Gate (removed for question 2) and remove the Locomotive from the tracks. With your hand, push one of the train cars with metal wheels and axles into the Activation section of Isolated track. When you do this, does the Gate now work correctly?
If you decide you do need more power, you have options. One option would be to continue using the power pack you have now to run just the trains on the track and use a second AC transformer, set to the recommended voltage, to provide Auxiliary power just to the accessories. The next step up would be to buy a more powerful AC transformer with one or more outputs to run everything. Eventually, if you continue learning and spending more time with this hobby, you might consider one of the more sophisticated control and power systems such as TMCC/Legacy and DCS. There is lots more to read about all this if you search this forum for what interests you.
If you still have questions about your specific issue, please start a new Topic on the Forum. You will get faster and more helpful answers if you include as much information about the issue including model numbers for each Set/Locomotive and accessory within your initial post. Describe in detail what you've already done to try finding the cause of the problem. Also attaching an overview horizontal/landscape orientation picture of your layout showing all the accessories in place and trains on the track, says more than 1000 words.
P.S. If someone on this forum helps you, please consider thanking them. They're probably not getting paid for it.