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... The transition is a trough (as opposed to a crest). ...

I've looked at some online VC calculators, but did not find them very useful for model railroading. The notion of what will "cause problems" has too many parameters, such as the length/wheelbase/overhang of your longest rolling stock, not to mention the condition and quality of your couplers, track curvature thru the VC, and such. In theory one could compute a mathematical limit but controlling all the variables in practice becomes unworkable, and is hardly needed in a model. Then there is also the matter of constructing something that conforms to the mathematically computed values (in three dimensions no less). In the end I just picked something that did not seem too abrupt and went with that.... IIRC, 4x-5x the longest car length.Ed

4x-5x the longest car per what? Per a certain percentage change? For any flat to a grade?

(Attachment Link) 2% grade to a french curve then flat for at least 1.5 car lenghts of the longest car, then gradual increase again to 2%

Sounds convincing. What's the 1.5 car lengths based on? Also I guess I'm still wondering how to determine the length of the curve

I just realized that I didn't answer your original question. Let's say we have a 50" vertical curve that connects a −2% grade to a +2% grade with no easements (to keep the calculation simple). A 2% grade corresponds to an angle of 1.1 degrees, so your vertical curve needs to subtend an angle of 2.2 degrees (= 0.038 radians) to connect the two grades. The arc length of a circular arc of radius R that subtends an angle of θ (in radians) isL = θRSo a 50" vertical curve would subtend 2.2 degrees in a length L = 0.038*50" = 1.9". This arc length is also - to a very good approximation - the horizontal length of the arc. So, if you have 6" to play with between your grades, you would only need a vertical curve of ~150" radius to connect the two grades. This should pose no problems at all for your trains.