It sounds like it would make sense,
But, if you start at the lowest on the “under” track, go around the loop to the top of the “over” track, wouldn’t that total run be a 4% grade?
Or am I missing something. [*-)]
Mike.
average grade depends on the difference in height and distance.
don’t see it. how can you “depress” the one track and still have the “same 4 inch separation”?
Starting at 0 level, go clockwise on a 2% grade going up. 100 inches later, you are 2" above the 0 point. Back to the starting point, run counter-clockwise for 100 inches, going down at 2%. 100 inches later, you are 2" below the 0 point. Total separation, 4".
If you start at the 0 point and only go up on one side (think your traditional figure 8 pier set), you have to use a 4% grade to get a 4" rise at 100 inches past the 0 point.
One track goes up, the other goes down, each at half the grade. The disadvantage is that neither track has any level space on it, they are both on grades.
–Randy
No, it would be double the run length at half the grade.
–Randy
But you traveled a total of 200" to get that 4" seperation. Evidently, the OP doesn’t have that much space.
Mike.
Well, we still don’t know how much space is available. In any case, we need to get away from the idea that we need to start from a flat table. Think of a subway/elevated train which starts at the elevated station, runs to the surface station, then tunnels underground to that station. The surface represents your table top. And for convenience purposes let’s assume that the stations are 100 inches apart, and that the elevation difference between stations is 2 inches
So the train starts from your surface station and travels to the elevated station on a 2% grade, which means that it has gone up 2 inches. Another train runs from the surface station to the underground station on a 2% grade, which means that it has gone down 2 inches. I think you’ll agree that the two trains are now 4 inches difference in elevation, but they have each traveled only 100 inches to get there.
Now if you wrap the two ends around toward each other you end up with one-half of a figure 8. At that point the trains will change grades to go in opposite elevation, at a 2% rate, to get back from where they started.
Think of your basic ‘train set’ figure 8 - some sets came with that track plan, all 18" radius curves, maybe 2 straight tracks for the center of the 8. Smaller than a 4x8. One part is completely level, the part that goes up and over is the only one that climbs, at a steep grade. If the next sectioon past the lowest pier didn;t stop at the table top and went lower - you’d have this effect. No extra space required. At, as I mentioned, the expense of having ANY track on the level, it would all be on a grade.
–Randy
This is referenced a bit in the original post too. I don’t think anybody ever said a 4% grade would cause derailments necessarily, or damage engines, or make them slide back down the grade. If your track is well laid it shouldn’t be a problem. However, a 4% grade will reduce the number of cars an engine can pull significantly. I seem to recall reading that a 2% grade could reduce an engine’s pulling power to like 40% of what it is on level track.
what’s the distance from 1) the surface station to the elevated station and 2) the surface station to the underground station.
i assume there is a 4% grade between where the tracks cross over one another or to achieve the desired change in height.
Ok visualize it this way. Surface station sitting at 0" elevation, there are 2 tracks at this station. 100" away are 2 more stations, one 2" avobe the level of the main station, and one 2" below the level of the main station. What’s the grade from the main station to the raised one? 2" over 100", 2%. What’s the grade going down to the subway station? 2" over 100", 2%. Neither train ever negotiates more than 2%. Now go another 100" down the line, and have station 2 at the 0" level. 2% down from the elevated station, and 2% up from the subway station. Again, neither train has ever negotiated more than a 2% grade, yet in the middle they were 4" apart.
Stick a balloon loop on either end, connecting the two tracks - a dogboone shape. Run just one train around the now continuous loop layout. It will go up a 2% grade, pass by the elevated station, descend a 2% grade, loop around station 2, descend a 2% grade to the subway station, then climb a 2% grade and loop around station 1. The train has changed elevation by 4" yet has never negotiated a grade more than 2%.
–Randy
ok, you’ve doubled the distance from 100" to 200" to change height by 4".
don’t understand how “depressed” track ends up being a longer distance
Exactly.
The depressed track becomes a longer distance because in the case of a single 4% grade, the other half of the track stays level. If instead of being level, it continues downgrade below the 0 point, you have extended the distance which has a grade in it, meaning you can reduce the grade for the identical vertical separation.
–Randy
100 inches between each station.
No, none of the grades exceed 2%.
In your example, to go between the two stations with 4" separation you have to go 200". That’s how you get the 2% grade. If you only have 100" of track and need 4" of separation, you must have 4%. Can’t overcome the math.
Ray
Exactly, agreeing with Ray. One train going from the elevated station, past the ground level station, and down to the below level station, has dropped 4". If this is done in 100", you have a 4% grade. If the train travels 200" to do this, it is a 2% grade.
No smoke and mirrors involved, just math.
Mike.
Right, no matter how you cut it, you are extending the run to reduce the grade. You cna do this in no extra space, considering again the figure 8 layout. Normally you have a track flat ont he baseboard. Then over 100 inches of track, it rises at 4% to give a 4" clearance. If the part that was flat ALSO was on a grade, now you have 200" at 2%, ot if you arbitrarily pick a point in the grade, you have 100" going UP at 2% and 100 inches going DOWN at 2%, either way, at the crossing point you still have 4" vertical, but there no longer are 4% grades anywhere. Or some split difference - change the 4% to a 3%, and make the part that goes below the 0 level use a 1% grade. You still get a total of 4" vertical, but now there are secontions with different grade.
–Randy
how can you extend the run in “no extra space”?
By not building it on a flat piece of plywood. The whole part of the trasitional train set figure 8 sits flat on the ground (rug, table, plywood, whatever you put it on). Instead of doing that, if you use some sort of benchwork, like l-girder with risers and cookie cutter roadbed, you put the whole thing on a grade. Now you have more distance on a grade, so you can use a less steep grade to obtain the same clearance.
At the expense, as I said, of any level areas for sidings and whatnot. But that’s applying the concept to a minimal size layout. It can be extended to bigger layouts as well. Say you have a double deck around the walls layout. At one end, you have a 4 1/2 turn helix to connect the decks, which are both completely level. If yoou start climbing on the lower deck before you get to the helix, and climb an equal distance on the upper deck past the helix, you can get the same deck separation with a helix of fewer turns. Extend it to the extreme and you have a nolix - double deck layout with absolutely no helix at all.
Just gotta think below the plywood.[:D]
–Randy
Wow…
If you wish to “split the difference” on a requisite separation of 4" in overhead clearance, you can make the underpass track descend for anywhere from 50-100" and then rise again to its previous level. The key for this splitting is that the overhead track must make up the deficit, which we hope will amount to, or total, 4" of overhead. This means, necessarily, that the higher level track must either climb by the same amount as the nether track descends at the preferred 2% grade, or already be at that level.
So, let’s say you have that figure 8 and start at the apexes of the two lobes, opposite where the crossing is. You don’t want a crossing…you want an overpass. So, pick one, but start at one lobe and begin an ascension, a positive grade, so that it rises about 2" by the time you get to the overpass. The point opposite, on the apex of the other lobe, must at the same time descend at the same rate. When you get to the overpass, the total accommodation in altitude with come to the needed 4". Then, the climbing track must commence to descend to meet up with ‘itself’ and the other must begin to climb for the same reason, and when they meet at their respective starting points they have a folded loop, figure 8, but with that oh-so-appealing overpass.
It should be obvious, then, that neither apex lobe point can be at flat rock bottom of the layout’s lowest permissible level, our you’ll need to do a lot of work to create a stable and supportive platform under the layout. No…you must start both lobe apex journeys at about 2-3" above what you may have intended. This might mean a 2.5" layer of foam that must be gouged down for the descending track, and then up the other side of the overpass. The other lobe starting point would have to use some kind of built-up structure, ramp, risers, to get up those needed 2". And so on…splitting th