My layout is mostly 18" curves with some 15" on inner city run. I recently purchased an Atlas Gold SDP35 and am having trouble with it staying on the tracks. I know it doesn’t like the 15" but thought it wouldn’t be such an issue with at least the 18" curves. It will make the rounds ok most of the times but occasionally either the front axle or the rear most axle will be off track. Now I am wondering if this is due to my layout not being a 100% level while under construction phase or would I be further ahead to make a run with 21" curves?
A 18" radius is tight for a six-axle Diesel, but it should be able to negotiate it slowly, without derailing. If there is a bump in the track, that could very well be the cause. If you have a chance to go for a bigger radius, do it - you will have less problems.
Laying smooth and level track is an art and takes time. I run mostly four axle locomotives but also have one SD9. I haven’t had any trouble with it on 18 inch curves, of which I have many.
I am using WS foam roadbed and gluing the roadbed and track down with clear latex caulk. I pin the sides of the track to hold it to form, and set some weights on the track to hold it until the caulk dries. I NEVER EVER fasten my track with track nails as this can pull it down and out of shape which can lead to derailments. You have to let the track flow and float some. As long as it is secured first, the ballast will take care of any gaps between the track and roadbed. If you try and force the track to lat flat on uneven roadbed, you will have problems.
I am not saying that is your problem, but it is a good starting place to look at.
This has been my way of describing how to think about engines and curve radius in any scale:
The closer you are to the minimums claimed by any of the manufacturers for a particular engine, the higher the probability of frustrating moments. As you force the equipment to get to its engineered lower limits (with variances in finish and assembly), you raise the effect on performance of other errors that would normally not be a problem. So, as Sir Madog quite rightly says, while 18" may be okay with really solid and careful trackwork, tiny imperfections become that much more the defining matter…and the engine gets to choose how it will behave, not you. [:-,]
Make sure your track joints maintain grade. One rail dipping or rising, and especially on curves (and I’m trying to make you see it triply especially on the minimal radius curves!) will cause many flanges to rise up over the outer rail, and the rest is easy to anticipate. The problem is that the rigid frame and the other truck coming to the derailment spot are wanting to keep the engine’s former attitude with respect to the track’s axis. This makes the leading truck able to do a lot of wandering when trackwork is less than ‘good’. Six wheeled trucks are a lot worse.
Please, do yourself a favour and figure out how to get another 2" of radius, more if you can manage. And check your tracks for level 12" on either side of the derailment location, and all points in between.
-Crandell
I’ll restate what Selector and others have said.
As radius decreases, the quality of track laying must increase.
For a 6 axle loco to work flawlessly on 18" radius curves, you must lay the most perfect track you can. Any bump will cause trouble. As radius increases, the margin for error does as well.
However, there is one other thing to check, get out an NMRA gauge and check all the wheelsets and verify they are in gauge.
A good habit to have is to assume everything you buy was put together roughly and that it is out of gauge, because it probably is.
I had a recently purchased BB Athearn SD40T-2 that was derailing on my clubs very broad curves and occasionally on a switch. It was caused by a couple of the wheel sets being out of gauge. I had taken it straight from the swap meet to the club. Now it runs much much better.
Most locomotives are made to run on an 18r curve. I wouldn’t expect a RTR 6 axle to be any different.
It can? LOL. I wish I had read this 7 years ago. I laid my track on top of WS Foam Track Bed on top of plywood and nailed it down, then ballasted. I had derailments forever even on 32" radius curves because of the unevenness caused by nailing. Finally, a few months ago, after my whining about derailments, one of our fellow forumites told me, “Fix the hump”. I did and the derailments ceased.
Rich
You should nail it down, but don’t drive the nail “all the way home.”
Oh, but they are, and the more coupled drivers on a steamer, the worse it gets. Think in terms of rigid wheelbase. Each axle added to a rigid frame extends the differentiation between the mean track level and the plane through which the axles pass. But don’t take my word for it, or the OP’s…let the trains do the talking. And that’s what this thread is about…trains doing the talking.
-Crandell
Part of the problem is that Atlas doesn’t advertise the minimum radius, or if they do it’s not obvious. Broadway limited does on their website under each product. I have about 7 of the Atlas Golds that run great but I do have 22" min radius.
I have to agree with you on the track/wheel gage check. I had problems with my Big Boy on 36" radius track and especially through a 36" radius turnout. Turns out the pilot wheels and half of the tender wheels were out of guage, and once I reset them, the problems went away.
Track work is critical and must be as good as you can get it, but don’t overlook the wheels, even on a new engine/car.
Bob
Don’t mean to be a wise guy but why didn’t you get the 4 axle units ?
You probably have some places where the outside rail of your curved track is lower than the inside rail, causing the outer-front wheel to slide above the outside rail. Check to see if the problem happens in the same spot(s). Look closely at the track 'cause it may be really tough to see.
The OP bought a SDP35 which is a 6-axle diesel.
I actually have compound curves which in essence are sharper than 18" and my F45 goes around them OK with a step clearance. The biggest hassle I have had is to make sure that the track is in gauge and that there are no bumps or very slight misalignments in the rail join areas.
The “intermittent” problem that you have is more than likely caused by what I would a describe like harmonic effect as the loco runs over the joint, the flange hits the joint at a given point of its shape or misshape and the rail gets picked and the wheels ride up, causing the derailment. A dip in the track exacerbates the occurrence.
I use Flex track almost exclusively but if you are using sectional track could you substitute a small piece of flex track between your last straight so that your locos truck “leads in” with a wider radius start? Using sectional track, you would lift the last 6" section of straight and the first curve and make a larger radius between the two end points.
My flex track is Peco and it can get out of gauge so check that your track is in gauge. This can happen even on sectional track where the track is pinned and causes the rails to cant inwards very slightly… it does not have to be much. Use spikes like you would with handlaid track through the plastic sleepers and recheck the gauge outwards.
As everyone else has said, the tighter your radius, the more exact you have to be but it can be done.
Good luck from Down Under,
Trevor
www.xdford.digitalzones.com for your interest
President Melton Model Railway Club www.meltonmrc.org.au
As already mentioned, perfection, perfection when laying sharp curves.
There is one other thing I have come across. Even with rail joiners engaged properly the height of the rails may not match exactly. I lay a flat piece of steel on the rails and slide it down the track both ways to see if it catches at the joints. Then make sure the rail tops are exact. My SD7, heavy weight coaches, and the 2-6-6-2 brought that to my attention. [:-^]
Good luck.
Lee
Thanks all. Since I was trying out the layout to see if it would be what I want I should expect these gliches. I am now more seriously thinking about going with more flex track esp in the curved areas which will someday be my mountain range. Maybe also I should see less problems once I get the roadbed laid.
My feeling is for turns 24" radius and under is to use the Snap-Track. Too easy with flex track to get a part of it a little too tight, and then stuff falls off.
One end of my basement table I had laid with flex track to 18" radius (I thought). 
The inner loop on the other end I used 18" Snap Track on.
I was able to get certain cars around the Snap-Track, but they fell off on the flex. I relaid the turn with Snap Track and the problem went away.
I do make sure the track is level, or the outside rail is slightly elevated. On the Snap Track I solder it all together, to avoid problems with it coming loose, or losing electrical conductivity.
It all boils down to track work. I do not use roadbed under my track. Just my personal preference and saved buy 500+ feet of roadbed. I use track spikes, not nail to secure my track down. Most of them are not tight or driven “home”. I also use a short aluminum level that is cut down to 2.5" long. I use this to level the track crosswise. I use 6" level to check the track lengthwise in short areas and 2’ and 4’ level for longer distances. Cross wise level is the most important. Next keep the high places out of the top of the rail.
[soapbox] I agree with just about everything said but I do have one question. I was of the schooling that the curves had to have a certain amount of super elevation. If this is not necessarily so it may explain a problem I am encountering on a new turnout where NOTHING stays on the track-locomotive or freight car. So I guess I’m actually asking as to whether or not curves should be level flat or slightly banked. Any debate?[banghead]