Minimum loop-back radius for long trains?

I am working on a plan for a long but relatively narrow layout that will have a double-tracked mainline coming into view from a staging are and going out of view about 45 feet away. The VISIBLE part of the mainline will have large radius curves (six FEET or so) that turn less than 30°. The storage tracks in staging are about 9' to 13'6" long, and I intend to run some trains about those lengths.

My question is how large should the radius be on the HIDDEN loop-backs in staging and at the other end of the layout? My concern is not how the trains will look, but whether they will get pulled sideways off the track because they are forming a 180° (or more) arc as they make the turn-around.

Would reverse super-elevation (leaning toward the OUTSIDE of the curves) have any value in preventing cars in these long trains from "stringing" off the rails?

I'll mention up-front that I am avoiding drag-inducing things like axel wipers and will ensure that all cars are weighted to NMRA standard for their lengths.

 

How large an area do you have for the turn back loops? Obviously, the bigger the better. My old layout had reverse loops that were 18 radius, and I would run normal train lengths of 12 to 14 feet long and have no problems what so ever. No reverse super elevation, just glued straight to wood subroadbed.

-Mike

 

What's the maximum length car and/or engine you'll want to run?
Body-mount or truck-mount couplers?
High-profile or low-profile wheel flanges?
What code track?
Curves with or without easements?

 

Also, will there be any grade to the loops? Quite honestly running 9' - 13 1/2' length trains should not be a problem using a minimum 15" radius regardless of all other factors. The question is are you wanting to use a radius less than 15"?

 

Thank you for your responses. They have been helpful. I have a 12' width limitation, and will have an 18" access along the wall behind staging. So, things start to get narrow for the layout on the OTHER side of the 12' wide room when I put in a 30" minimum isle, unless I go down to 19" return loop radius for staging. (I had started drawing with Unitrack #6 turnouts and 28" radius loops, but redrew it with 19" radius.) There MAY be a 1% grade approaching the loop on one end; I am still planning. I can put in easments with flex track. Wheels are what comes on a large variety of cars, with change-outs of whatever misbehaves. I tend to put BLMI or Fox Valley metal wheels in original trucks as the first step, then go to better trucks if necessary. Track will be code 80 where not visible, and code 55 where it will be seen. Equipment will include 85' passenger cars and E-units, plus an EM-1 (2-8-8-4). The longest train with 85' cars will be between 7' and 8'. The EM-1 will probably be pulling the longest train of 2- and 3-bay hoppers, although a pair of FT diesels may rival that with "Time Saver" fast freight using mostly 40' and 50' box cars. Those train lengths will be limited by the staging area storage track lengths. Couplers are a hodge-podge of knuckle types with both body and truck mounts.

From the responses so far, I am reading that 19" loop-back radius should be headache-free for operation.

I do plan to make a car test rig for my work area that will allow me to check the grade that will start each individual car rolling. That will allow me to find the cars that create maximum drag. I will be working on those first to make the trains as long as I need, and that might also help me identify if a car with high drag is the cause of any "stringing" problems, once I get a layout to the track testing stage. But, I want to be testing a track plan that SHOULD work for the car sizes I will run, and then focus on tuning cars, rather than ripping out just-laid track because I am trying to squeeze-in too sharp loops.

Thanks again for your advice. And happy holidays to all.

Steve

 

I ran a 64 car consist on the club layout with 4 Life-Like FA2/FB2 units through some 15' to 19' radii curves. The real problems occurred when the consist was not distributed with heavier cars closest to the motive power... and lighter cars at the end. I had some under-weighted rolling stock in the center of the consist which were string-lined at the upgrade of the 15" radius. The same consist went through the 19" radius downward grade without a problem. I think the combination of added stress on the upward grade, the narrower radius (15" radius 180 deg. turn) and light cars in the middle was a recipe for disaster. I didn't try going downward the 15" radius nor upward on the 19" radius to see what results I would get...

 

Thanks for the experience, Joe. I have seen some string-lining problems at showes with NTrak layouts, where they were trying to pull 60 coal cars out of staging and onto the main around 180°, but flat and with much larger curves than I hope to use. That is what started me worrying about my 19" radius plans. It makes sense that down-grade helps and up-grade might cause problems. My staging area WILL be flat. But, I am still considering making part of the mainline where it goes into the loop-back (at the other end of the layout) a few inches higher so that my short-line can pass under it (hidden by a tunnel) instead of going around it. That would mean grades approaching the loop-back, with much of the longest trains still being on an up-grade when the engines have completed the loop. (Pi x 19" = only about 5' of train actually on the 180° curve, and maybe up to 8' still dragging up the grade.) Still, if I do go for a grade on that end of the mainline, I will limit it to 1%, for that and other reasons.

Steve

 

I run 22 cars behind a single loco on 11 inch curves, never had it stringline. Some of the cars are very underweight E&C shop hoppers. I use mt trucks with attached couplers. I suspect if you take care during track laying that the rails are flat and in gauge you will not see a problem. For me that involves soldering flex track without the bend.

 

I've been using 13" as an operating number for years with dead-flat reverse loops and 25-30 car trains, including one that's all piggyback flats. You're pushing it with body-mounts at that point.

I designed my staging yard as essentially a big lower-level nolix with one entrance and one exit, and a loop mentality to get out. It's been very reliable. I have a diode matrix to set up entrance switches to 8 tracks, and the only way you get a green route light is if the route clears and all the switches lock. On the outbound track switches, the points just float free so you can't go through a locked switch and derail. As it is unidirectional, and DC, there's a diode on the power supply with an emergency pushbutton bypass - you can't accidentally reverse out of staging unless you deliberately want to back up for some reason.

Biggest glitch you want to plan for is if you design in reverse loop - the placement of the insulating gaps. Because you WILL periodically hit those gaps, even with detection systems and warning lights, and if you slam a train into dead stop on a short and then hit the polarity main toggle and it slams forward, that's the exact moment when you'll stringline if you're going to stringline at all. I put my gaps way, way out so that as much of the train was through the loop curves as possible, and if I'm not paying attention, I can still create an epic disaster anyway. I simply don't do running through the hidden loops steadily anymore, you come to a complete stop when the detection system sees you outbound, then throw the switches and change main polarity, and restart. Don't be a hero.

I had a good friend that used run a 90-car train of MT 2-bay hoppers through a 12" reverse loop behind a Roco 2-8-8-2. Cars were light and short but he never stringlined unless he'd slam into the polarity insulated joint and then throw the toggle. Other than that, flawless.

 

Thanks, randgust,

I am designing the loop-backs for a double-track mainline such that there is no reversal of polarity in normal operation. In the staging yard, there will be 2 loop-backs capable of moving trains from one mainline to the other and reversng their directions. But, on the other end, the mains simply go around the nested loops and head back to staging hidden behind scenery. So, reversing loops can be used only when wanted AND WATCHED in staging. Continuous operation for display would not require any reversals, nor would swapping trains in and out of storage tracks so long as they go in the same direction.

Steve