When I was describing my new RS3 .ocomotive, I had mentioned that it had some issues with vertical curves or elevation transitions. I guess elevation transitions is the correct, engineering term but I’ve heard far more in model railroading circles about vertical curves so I guess one can use the terms interchangeably.
The drawing above is exagerated but shows the problem created by an abrupt change in track elevation. In A, the track is level and the couplers are alighned properly; there is no uncoupling. But in B, you can see that the unit on the left, could be a loco or another car, has already started down and the rear of the car raises up. When this happens, the couplers are not lined up correctly and there is a break. In C, the first unit has started up the incline and the rear drops this time causing the same result. Note that B and C could also depict transition to level track which causes the same effect… uncoupling. Keep in mind that these drawings are done as extremes to more easily show the cause of the problem.
This problem is particularly noticeable in locomotives where the loco trucks are more towards the middle of the unit and there is a longer overhang on the front and rear of the loco. This makes the distance from the tipping point, the loco truck, to the coupler longer and exagerates the change of the curve. Typically, steam locomotives do not have this issue because the coupler is located at the end of the tender and the rear tender truck is near that coupler. I have seen it though on some front couplers of switch engines where the coupler is again located quite a distance from the first driver.
It has also been my experience that four axled trucks tend to have a higher likelihood of this occurring than six-axled ones. Again, I believe this is because the longer trucks, the six axled ones, are located closer to the end of the loco. With my own loco fleet, there are only a couple of locos where this problem really rears its ugly head. Both of them are four-axled units with relatively short wheelbases and longer “porches” on each end.
There are basically two ways to try to resolve the problem. One needs to be addressed when laying track. As you might guess, that is simply laying track in such a way as to allow for a longer transition when beginning or ending a grade. If I had it to do all over again, I would try to lay a 36-inch piece of flex track at the beginning and ending of each grade so that the actual starting point was right in the middle of the piece. This would give the track a much more shallow bend and probably go a long way to solve the problem. As it is, the offending spots on my layout are at places where there is both a change in track elevation AND a track joint. While it can’t easily be seen, I suspect that this has resulted in a pretty sharp uphill or downhill bend at that point. If one were to round off those transition points, especially at the top of a grade, that would help a lot as well.
The second option may be the installation of “shelf couplers” on the offending locomotives. Shelf couplers are simply regular couplers that have been modified to not allow the coupler heads to slip up or down and cause a coupling break. As you might guess, these couplers have a small shelf on the bottom and the top to prevent this vertical motion. Once the coupling is made, the couplers cannot slide past a certain point, keeping them coupled much more securely.
As always, there are downside risks. With the transition trackwork, there would be a lot of work required to tear up the track and round off the transitions. With the shelf couplers, the vertical movement is restricted so, if the track work is really bad, the lack of vertical movement may actually lift a trailing car right off of the track resulting in a derailment.
It is my honest opinion that the modeling press “generally” does not talk enough about these vertical curves. I have read plenty of layout design books and articles over the past 35+ years and still didn’t really find any reference to the problem until after my track was laid and the problem started showing up. Had I known ahead of time… the construction techniques I used would have been a lot different. Model railroading is fun… and can also be a learning experience!