Examples from two different viewpoints of long 85 ft passenger cars running through the 24" long computer generated transition curves from the 48" radius end section of the new, under construction, PROTO:87 STORES TEST AND EXHIBITION LAYOUT.





Note that the car ends line up to within 1/8" throughout the transition from curve to straight, with a optimal smooth sideways motion and no wobble.

This is the main purpose of transition curves on a model railroad.

First they provide the cosmetic realism appearance of long cars staying smoothly well aligned, even when moving on and off the relatively extremely sharp curves that most model railroads have to use to fit within a small space.

Second, this helps functionally in keeping working model couplers and coach diaphragms within a safe working degree of sideways movement, and thus prevents forces which may otherwise cause derailments.

Since this is a 4 track main line, the use of a computer ensures that each transition curve is 2" radius larger than the one inside of it.

The frequently mainstream hobby promoted "bent stick method" of using a piece of springy wood or "the naturally free curving position" of some flex track, is really only a way to approximately "smooth out" obvious curve end and mixing differences that otherwise stand out visually in sectional track like laid layouts. The quite reasonable good reason for doing that is that the track looks much better visually and it helps reduce unrealistic sideways jerks in car movements. However, while using a bent stick (actually a springy straight rod is required) theoretically conforms to a spiral if under one end pressure, it does not provide any information to the modeler on the correct extent or position of the necessary offset of the straight track end spiraled from the regular curve radius center, or the transition length. Nor what the "ideal" or reference bent stick (rod) material, size or length is. So any two modelers using the bent stick approach to "transition" the same curve problem, will almost certainly come up with two completely unpredictable different results. And so that's not a reproducible result "method" at all.

The track planning software I used was called 3rd Planit. I think it is still available, although my version is many years old. You don't have to calculate the easements. The SW fits them in automatically as you draw your plan and plots the corrected track positions, including the offsets. My requirement was to optimize (minimize) the relative end movement of long passenger cars going from straight to curved track. Since an 85 ft car is already about 12" long, it is fairly clear that a transition curve 12" long or less is not going to have much, if any, effect. So I chose what I think was the "long" transition option in the software, which turns out to be 24". The results as you can see from the video are quite presentable and realistic. Ideally, the the car ends will stay closer together if the transition curve is longer, or as long as possible in the space available. But this was a first test, and I didn't want to have all transition and no fixed radius in my end curves. If I ever want to relay the track, I think next time I'd try 36", but that won't be anytime soon :).