In the real world, train derailments are a tragic thing. Damage is not only done to the train, but to the surrounding area, and people can get injured or die. This is why railroads have precautionary measures in place to prevent this from happening. Likewise, train derailment is an annoying problem common for both novice and advanced model railroaders. It's not as serious as a real train derailment, but it can cause damage to your locomotive and rolling stock. There's nothing like getting your track laid the way you want it, only to find there's a problem when you try to run your train. I'm going to give some tips on how to find the source of the derailment, and how to fix it for good.
The easiest thing to do to fix your derailment problem, and probably the most effective thing, is to check your track. Make sure there is no debris from scenery, ballast, or other objects that are on or between the tracks. Check all the joints of the track by running your finger along the track and fixing any gaps made by joiners that aren't secured properly. Some modelers solve this by soldering the rails together and filing them smooth so that the layout has continuous rails throughout. I don't recommend doing this. If you solder all the rail gaps, you leave no room for the rail to expand and contract. If sun shines through a window onto your track or the temperature rises during the summer, your rail will buckle because it has no room to expand. One more thing is to check if the track is even. Uneven track can make bumps or rises on the track, which can cause a wheel to jump the rail; therefore causing a derailment.
This could be the problem if your train always derails on the same curve. When building your model train layout, it's important to know that not all curves are created equal. If the curve you designed is too tight for your locomotive, or rolling stock to navigate, it will derail your train. Test this by first running your locomotive at a moderate speed around a curve, and if it passes that test, add cars one by one to see if one of them is the root of the problem. Also, check the manufacturer's instruction manual to find the minimum curve recommended for your train. If either the locomotive or rolling stock is causing the derailment, then try to run the train at a slower speed. This will keep the centripetal force on the train low, and may keep it from derailing. Otherwise, you will need to relay the track with a higher radius curve.
Check the wheels of your locomotive, and on the trucks of your passenger cars and rolling stock. If any of the wheels have a bend to them, even slightly, this could cause the wheels to be out of gauge and cause a derail. Also, check for any squeaky sounds in the wheels, this is friction between the wheels and the truck. This friction can cause a jar on the car that makes the wheel climb the rail and derail the train. Fix the squeak by adding a small amount of oil to lubricate the wheels.
These are commonly overlooked. Some couplers on new rolling stock can come unpolished with edges that can snag. This snagging on the couplers is enough to derail the train. Check your couplers for any rough edges, and smooth them out with a filer. Also check the height of the couplers. If they are too low, they may not be able to clear switch points or other track parts, and these can knock train off track. Fix the height by adding shims to the bottom of the car to lower couplers, or washers to the truck in order to raise couplers.
This problem can be caused when you are laying your track. If your train keeps derailing at a specific spot, check to see if that spot is where you nailed down the track. It is common to nail too far and break a tie, or leave a tie on the verge of breaking. When this happens, the pressure on the tie will pull the rails inward, making the space between them smaller. This in turn causes the wheels to climb the rails and jump the track. Use an NMRA standardized tool to check the gauge at that spot, and at other problem areas such as turnouts and frog assemblies.
If your train is derailing at a switch, check if the frog and guide rails work smoothly, and are clean of any dirt or debris. Next, check the switch points. Look for any blunt or rough areas where the wheels can grab, and file them. Switch points should be sharp and smooth against the rail, and the gauge should be the same with the switch in both positions. Afterwards, run a spare truck across switch by hand to check for instability.
The good thing is that you don't have to shed any pounds, but adding a little weight may help. Sometimes locomotives and rolling stock can be a little unbalanced. This uneven weight distribution can give cars a slight lean that makes it easier to derail when going around curves. Adding weight can undo this imbalance; ensuring all wheels come in complete contact with the rails, and distribute weight equally across the car. Follow these tips and you will prevent derailments from causing damage to both your layout and your train.