ok, you've doubled the distance from 100" to 200" to change height by 4".
maxmanOn the other hand, if the track to be crossed can be depressed below grade by 2 inches, then the track crossing over only has to go up 2 inches. We end up with the same 4 inch separation but the grade for each track is only 2%.
don't understand how "depressed" track ends up being a longer distance
greg - Philadelphia & Reading / Reading
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
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
gregcwhat's the distance from 1) the surface station to the elevated station and 2) the surface station to the underground station.
100 inches between each station.
gregci assume there is a 4% grade between where the tracks cross over one another or to achieve the desired change in height.
No, none of the grades exceed 2%.
rrinker 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 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%.
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.
My You Tube
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.
rrinker 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.
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.
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 the difference.