When is a reverse curve, not a reverse curve?

Basically, yards and industrial spurs are an exception as they're taken at very low speeds. A crossover between two tracks is designed to be taken at normal speeds, though some are designed to be taken more slowly.

[Opinion Here] There is no truly "acceptable" reverse curve unless there's a straight long enough to handle the largest car is in-between. That said, you want the broadest curve possible if you're going to do a reverse curve as it will mitigate the problem. Atlas has 54" radius (O-108), Ross has 64" Radius (O-128) and Gargraves has 69" radius (O-138) available. Lionel stops at 42" radius (O-84) and MTH stops at 41" radius (O-82).

Operationally, if you don't take them too fast, you won't have problems with most 3-rail equipment with truck-mounted couplers. But the shift in lateral momentum from the reverse curve will eventually cause derailment as speeds go higher. Larger steam locomotives with long driver wheelbases can short as the drivers may touch the center rail on a reverse curve. Hence my saying "just because we can  doesn't mean we should." Aesthetically, it also looks better to have straights in between reverse-direction curves.

Larger radius curves tend to be more forgiving, and low speeds lessen the impact of a reverse curve during operation. i would still recommend breaking up any reverse curve with a tangent where trains operate at speed--for cosmetic reasons, if nothing else.

Jeff C

Because in switching yards the trains are moving much slower, are being actively watched and the consists are shorter. Spurs can be the same way. IMHO, size doesn't matter as much as how you operate your trains, especially speeds. Unfortunately, too many buy an R-T-R set for Christmas that has O27, O31 or O36 curves and then naturally assume the same size turnout will work. Then they see little Johnny running at full speed through the turnout and derailing. I think most would recommend the largest you can get, but that often depends on what size your layout is and what brand track you are using. I have no problem with my O31 turnouts, but then I operate at more realistic speeds and understand the potential for problems. Anytime you can avoid a reverse curve, you are better off. As you have seen, there are ways to use standard switches and avoid reverse curves.

Well said!

There are different ways of looking at this--when we are talking about curves (including reverse curves) in a track plan three basic factors come into play:

will equipment operate through the curve

will the curve fit in the available space

how will the curve affect the overall scene--in other words, will the curve negatively affect the overall "look" of the layout, will the equipment owned look silly going through the curve, and will the overall space taken up by the curve make the overall track plan unworkable, or not worth the effort.

The first two of these points are pretty much non-negotiable, but the third consideration will be the point of contention, where different people come up with different answers, based on what they consider important.

Jeff C

What is a reverse curve?

Alex

I usually here it referred to as an S curve on the forum.

It's when a curve is immediately followed by another curve going in the opposite (reverse) direction. Like Tim says, it is more commonly referred to as an "S" curve because it forms an "S".

Here are some of my preferred yard ladder track arrangements.  These avoid "S" curves. Shown are O-54 turnouts with O-72 main curves.

The O-54 turnouts create ladder tracks at 30 degree angles with about 7-1/4 inch spacing between tracks.

When modeling an S curve remember to put at least one straight track 6 to 10 inches long between the first curve and the opposite curve to avoid derailments.

Jack 

Generally, this is a problem for small layouts where you are "space constrained" and trying to do as much as you can with the space available in which case you may have to live with the S curve.

I agree, it's better to avoid them if you can.

If you have the room, you can add the straight sections between the two curves to minimize the "S" effect, but this will eat up real estate. (Top of illustration)

The other alternative is to go with # switches (i.e. a Ross std 11 degree) as shown at the bottom of the illustration.

I found on the main line (higher speeds) it was necessary to have a straight equal in length to the longest piece of rolling stock between curves going in opposite direction for best operation.