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I’ve been going through a “carpet central” phase, and experimenting with various curves. 

First, using tubular track, I’ve been experimenting with O54 and O42 as simple and transition curves, which seems to be a good combination. Greatly reduces cab overhang and also suits my modified Williams Scale Hudson - the 17mm coarse Scale (ie not Hi Rail profile) front pilot wheels manage this easily. 

Secondly, Fast Track. I’ve now sourced some O72 to complete the curves. This is definitely the best size for O Gauge! I don’t presently have room for an O72 loop, though. I also have a full circle of O48 and O36. The Hudson will negotiate O48 well enough, although it won’t handle O42 for more than a couple of sections.

I’ve presently got a loop with ends as follows - O72, O72, O36, O36, O72, O72 and that works surprisingly well. The Hudson manages the O36 section, which the large front pilot wheels won’t handle an O36 semicircle (the original wheels handle it without problems, of course). The key seems to be, not to have a full loco length of O42 or smaller. 

I had fully expected that I’d need to introduce O48 transitions - something like O72, O48, O48, O36, O48, O72 - but it hasn’t proved necessary. However I think I’ll try it, to see the effects on cab overhang on the middle section.



Original Post

Make sure you.include one car in experiments into undersized curves. I have two that can make tighter than designed curves ...ALONE .  Cars get dragged off because of pilot overhang/coupler ends up traveling wide too, and the angle between 2 couplers binds before the angle of the turn is reached.

On those locos, to run in the tight curves, my lead car will have an articulated coupler shaft like on passenger cars. The articulated pivot point is moved towards the coupling point relieving some of the angle at the couplers.

I think the only real way is to experiment with what works for the locos you want to run and the specific benchsize and shape and configuration of track loop you want to build.  I built my layout fifteen years ago with Fastrak in the way you did, blending in transition curves in places and mixing and matching curve pieces of different radii.  It worked well. 

However, about seven years later I in stalled Atlas solid rail track in its place (due to a combination of Fastrak being too loud when on a benchtop, compared to when on the carpet central floor, and corrosion on my existing Fastrak due to my cleaning it with Simple Green, which cleans well but gradnually eats away at its stainless plating).  I used only Atlas Flextrack, and a little study and geometry, to vary radii and 'blend transitions' well and smoothly as I wanted, and it worked wonderfully.  It did take some effort and study, but it was worth it. 

I have a three or four car train of lightweight MPC era and modern stock, with a lighted caboose on the end. This soon shows any problems with string-lining etc. and I don’t seem to have those problems here.

The Hudson is definitely the limiting factor, with its long tender and oversize front pilot wheels. However the key seems to be, minimising the deflection. As the pilot is on the O36, the loco is already turning on the O72 section - it won’t handle the transition from straight, to O36 directly, and even straight to O42 is pushing it. It will handle simple O48 curves, though, do the critical value is obviously somewhere around 45-46”.

If I had the space, I’d definitely go O72-O48-O36, it looks better and further eases the deflection. 

No matter what track system you use, "easement" curves are definitely the way to go if you can get them to fit in your layout design.  The real railroads use easements too, for performance of their full size trains.

Case in point - the largest oval on my MTH RealTrax layout uses O82-O72-O54-O54-O54-O72-O82 curve sections at each end of the oval.  The width of the oval is just a tad over 5'-0" center-to-center, or just a little over 5'-3" out-to-out over the gray roadbed.  Trains not only perform good, they also look good as they as they traverse around.  It's almost as good as having an oval using nothing but O82 curved sections, but with a considerable space savings. 

Like a few others have stated though, you just need to be sure that any equipment you have can negotiate the smallest portion of the curve trackage that you have.  Believe it or not, by using easements, it's possible that some equipment can negotiate smaller than the manufacturer's recommended minimum curve size.  What's not to like about that? 

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