Hi Jerry,
You have been given some good information so far and hope you have solved your problem but it can be a little trying/frustrating to say the least but thats
the reason for the forum is so the knowledge can be spread around to help as
needed.
There are a number of reasons the Loco/cars could be derailing and wheel spacing could be one of them as well as the rails are too close together or too far aprt to allow the guard rails to do their job properly [ I'll explain below], so a gauge can be an important tool to have. I have them abailable through my Foot Hill Supplies under, For Sale & Wanted should you want to deal with me?
The way a switch is made the engine and cars tend to want to get misaligned
as they pass through the Frog, the central portion where the two rails form a point and the actual Points or moveable rails, specifacly where they touch the outside rails, guide the Loco and cars through there to head in the desired diection which of course your aware of.
But there can be problems that exist in that the Guard Rails on either side along the outside rails and opposite the Frog don't pull the the wheels far enough to the outside which can allow the inside wheel to climb up on the Point of the Frog and derail. Atlas switches at one time allowed too much space between the Guard Rail and the outside rail and a shim of somewhat thin plastic had to be glued to the guard rail to pull the wheels toward the outside rail so the trucks wouldn't pick the central point.
Also as previously mentioned it's essential to ensure that the points press against the outside rail somewhat firmly to ensure the the wheels don't have a chance to pass between the point rail and the outside rail as this will also cause a derailment.
Another common problem is in tacking the track, Tie/s, down too firmly against the board which causes the tie to start to take on a V shape which pulld the outside rails together making it narrower or a tight spot and a cause for problems.
Depending on the Loco you have, as has been mentioned, the sharper radius of he switch may be too sharp for the loco. Also if all wheels are flanged that decreases the flexibality of the loco to negotiate a tighter raduus. So keep that switch for a yard where you'll only be operating a smaller swither which shold handle the smaller radius just fine.
Also your going to find that longer cars, if it's your intention to run them require at least #6 if not prefferably #8 especialy for the mainline. Now if you don't have the room to use these longer switches you may need to switch to shorter cars that will.
Another thing that was mentioned is that tender and car trucks should have some wiggle room and not be tightened down to the underside of the frame to make the car or tender inflexable to side-to-side movement a little bit at east as that alows for smooth rooling over inprefections in the rail and making adjustments in alignment when rolling through a switch.
Also as was mentioned, if your using Kdee couplers or similiar with a metak shank the curves down it is possible that if they are not adjusted properly they can hit or rub against the rail which will cause the car/loco to be pushed or pulled one way or the other and that can also cause a derailment.
Another possibility is improper adjoinment of the rails which will cause one rail to be either higher or lower than the other which again can cause problems.
So I'm sure by now your begining to see there are quite a few different possibilities for your derailment problem.
Hope this has helped and not too difficult to understand.
By the way, Mark and I build our own switch using templates so we come upon these problems quite often, with a little practice you'll become an old hand at track laying and be guiding others. It's all part of th learning process.
. Just to check you are using something to switch it like the atlas switch machine right? (You be surprised the people I have met that switch it with their finger and are like why is it moving halfway when my train go through.
