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Clearance and Grade
With this, we move just a bit beyond beginner and toward intermediate. It is good useful information for understanding some of the things needed in designing your pike (layout).
Our models, like the prototype, have limitations on where certain size cars and locos can travel. How close to the track should a station platform or team track platform be? How high does a bridge need to be to clear the trains underneath it? How big/tall should a tunnel portal be and how wide? How close to the track can you have a ground throw switch stand before the cars hit it?All of these questions are matters of clearance. The N.M.R.A. took their lead from the AAR (Association of American Railroads) Mechanical Division and designed our clearance template and clearance gage to be very close to the real thing. Yes, there are a few differences because we are working with models, but the shape of the clearance template is very close until you get to the most modern diagram for double-stacks.
(There is a page explaining the N.M.R.A. clearance standard and clearance gauge by showing you the AAR Plate Diagrams. You can read this at Standard S-7 Clearances and the NMRA Gage.)
How does this affect you? For the simplest layout without grades, tunnels or buildings close to the track, not very much. However, as you begin to think in terms of hills, bridges and tunnels, you need to realize that the width of tunnels and the height of bridges above the track become important and very dependent upon what kind of trains you want to run.
I can tell you that most pre 1960 cars and locos will pass under a bridge that is 3 inches above the track in HO. They will also run through most ready-made tunnel portals quite well whether single or double (two tracks side by side) portals. I can also tell you that modern double-stack trains will hit that bridge and 'maybe' make it through a tunnel portal, but probably not. Short freight cars will take 18" radius curves quite well, but long ones may hit buildings and telephone poles set close to the track to the inside of the curve. The sharper the curves, the further away from the track things need to be. It's called Clearance!
Hey, how did we get that bridge 3" above the track assuming that it is a railroad bridge? We had to go up a grade to get there. Now, we look at how long it took us to climb and that becomes percentage of grade. Do I recommend the train set trestle set to get up and over? No, not really because the temptation is to climb too fast. A very good grade would be in the neighborhood of 2 percent. That is to say, a 2 inch climb in 100 inches. Ok, you don't have that much space? My own grades are 2.5 percent to 3 percent. The sharper the climb, the less cars your locomotives can pull. Most steam locos won't pull as well up grade as diesels.Running upgrade on a curve adds to the effective grade. What started out as a 2.5 percent climb may actually be 3 percent or more depending upon the radius of the curve. There are formulas for calculating this, but I am not going into that now. (Whew!)
If I say that a railroad bridge needs to be 3.5 inches above a track running under it, how can I get there without increasing my level of climb? Ah, the same way the prototype does it. Lower the track under the bridge! No one said that the lower level track had to be running flat and level. Think outside of the box. This is exactly where understanding something about clearances and grades come into your planning.Last item. Just as when laying curves it is good to have an easement to get into the curve more gradually, with grades it is very important. You need to ease into the climb and you need to lessen the grade as you reach the top. Can you imagine the coupler on your loco hitting the track as it starts to climb upgrade or dropping down onto the track as it reaches the peak? Been there, done that. The sharper the grade, the more important easements become.
- For additional information see:
- Standard S-7 Clearances and the NMRA Gage
- S-7 . . . CLEARANCES
- RP-2 . . NMRA STANDARDS Gauge
- For N.M.R.A. members there are Data-sheets available in hard copy and on CD as a cost item covering many practical tips and applications of the N.M.R.A. Standards and Recommended Practices.
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Page last updated December 27, 2000