FAQ - Benchwork

Frequently Asked Questions


What do you use for roadbed and subroadbed?

It depends where the tracks are located.

For tracks out in the country side I lay track directly on masonite spline gluing down flex track with paintable caulking compound (spread thinly).

For city area tracks I use 3/4" plywood (often Home Depot birch) with cork roadbed on top - 1/4" for main tracks, 1/8" for sidings, or track directly on plywood for spurs. I don't see much advantage to spline roadbed in places that will be flat and have lots of track. Too much work to get the spline perfectly parallel.

What is a Bellina drop?

A Bellina Drop is a style of backdrop named in honor of the late Jerry Bellina whom was a pioneer of its use.

The theory is that being able to stand at the end of a peninsula and look down both sides of it at once is visually jarring and leads to those dreaded model-railroad-thoughts. As in "since when can one look at the sky 'end-on'?".

Also, looking at the outside of a (most likely tightnest radius) U-turn curve isn't very attrative. If the railroad is set in the mountains one can always scenic the turn-back with a cliff or two and meandering creek. Or the cliched curved wooden trestle plunging dramatically down from the track.

But for those modeling flatter territory there are not many options.

Jerry asked "Why not put the backdrop on the OUTSIDE of the peninsula's turnback blob extending it far enough down the peninsula on both sides so one is not able to stand adjacent to that nasty blob and see all the way through to the aisle on the other side.

The backdrop scenery is painted on the INSIDE of the Bellina Drop. The outside of it is painted as if it were the fascia (which in effect it is).


  • You can no longer stand at the end of a peninsula and look down both sides at once.
  • You don't get treated to the sight of long equipment struggling around the too-tight curvature present on most turnback curves (an inside view is much nicer).
  • You can't see across the peninsula anymore (unless it has a see-over backdrop.
  • It divides the peninsula into two different vignettes making the run seem longer.


  • Have fun painting the backdrop on the inside of one of these...
  • Care must be taken in ighting the blob to prevent weird shadows from showing on the back drop(s).
  • The appearance is untraditional enough that it may be offputting.
  • You lose access to the track and space inside the blob. It's probably not going to work to stick a turntable and engine facility inside a Bellina drop.

What is a helix?

A helix is track laid installed like a corkscrew. It is usually used to move trains from one level to another of a multiple deck layout. It is often used when there isn't sufficient room gain enough altitude to get between decks.

For example, you have a 20'x10' room with an around the walls design. Your design calls for a level at 40" and another level at 60". To cover a 20" change in elevation at 2% will take 80' of track (1.3x the perimeter of the room). This isn't looking good.

But with a 36" radius helix: 3' * 2 pi = (roughly) 19' per lap so with 4 loops on the helix you're about there. The helix will occupy a big chunk of space (about 6'x6') though.

What's the separation between helix levels? Let's take that 19' and multiply it by .02 (2% grade) = .38' = (roughly) 4.5" or rise per loop (@ 2%). Subtract .75" for the thickness of the roadbed and track and you have 3.75" of headroom above the rails. This will work pretty well in HO.

What is induced drag?

The late John Allen theorized that when a train is on a curve there is extra drag. The drag is proportional to the radius of the curve and to the distance between the rails (the gauge).

This happens because the inner and outer wheels of your rolling stock are fixed on a common axle so they must both rotate at the same rate. But the inner wheel has less distance to travel around a curve than the outer wheel does. With no differential on the axles something has to give. One or both of the wheels start to slip. In the process of doing this extra drag is created.

On most model railroads the curvatures are quite sharp so quite a bit of drag is created. But most of our curves will only hold a handful or cars. But on a helix this is not the case. An entire train may be on the curve at once.

When this happens significant induced drag appears as though the track were steeper than it actually is. For example, with a 30" radius helix in HO you can expect to see an apparent increase in grade of somewhere near 1%. That means that instead of a train on 2.5% grade it will appear as though the grade was 3.5%. Yikes!

Should I use a helix?

There are a few questions that need to be answered before starting construction of a double deck layout with a helix between the decks.
  • How far apart are your decks and how large a radius can you use in your helix?
  • Where do you need the ruling grade on your layout to be located?
  • What scale (N, HO, O, etc) are you modeling in?
  • Will the helix be a part of every mainline trip?
  • How good are my carpentry skills?

Helix radii and considerations

All numbers are approximate. Assuming
  • 3/4" for the thickness of roadbed and track.
  • Assuming 0.5% induced drag.
Min radius
Circumference at min radius
Required overhead clearance
Grade for minimum overhead clearance
Apparent grade with curvature allowance

Note: Minimum clearances from NMRA standard S-7. These may not clear double stacks!

Helix location counts!

If a helix must be located in mid-run stop and consider. What percentage of the run will be spent getting dizzy in the helix?

For example:
A 36" radius helix in HO with an unadjusted grade of 2% climbing 4.5" per lap requires 4 laps to climb 18". That's 76' of track in the helix. In our previous example of the 10'x20' room a reasonable amount of track per level would be about 50' (allowing for curves in the corners and not counting the space occupied by the 6'x6' helix area). With two decks that's a total of 100' of track in the open with 70' in the helix. Assuming a constant speed you're looking at a run that's 41% in the helix.

Running trains in a helix isn't very exciting. And over 40% of the run is in a helix. If the runs don't use the full 50' on each deck the ratio gets worse. So the helix has reduced what Joe Fugate calls the quality of run.

In general...
  • A helix shouldn't be in the middle of a run.
  • It's more acceptible if the helix connects to a branchline
  • It's more acceptible if the helix connects the main levels with staging
  • A helix should NEVER be completely hidden. Crews go nuts when they can't see their trains for long periods of time. 70' in HO is 1 1/6 scale miles!
  • Don't put more than 50% of a run is in a helix.
  • Make sure there's enough space for fingers in a helix - for when things come off the track.

Can you build it?

A helix is a non-trivial piece of benchwork and track to construct.
  • The grade needs to be constant over a long distance.
  • The roadbed shouldn't rock from side to side.
  • The track must be reliable electrically - since there will be access problems to track in a helix.
  • The track must not be kinked at railjoints or rolling stock will derail. Test each lap as its built with you most finicky locos.
  • For a double track helix ensure there is enough track to track separation for ANY equipment you might ever consider running!
  • Remember to leave access to the helix for maintenance (and rerailing)
A decent helix isn't an impossible project, but it's construction will require some care.

Is your helix your ruling grade?

If your helix is the ruling grade on your railroad you have two potential problems:
  • Trains may fall-down (stall) in a relatively inaccessible part of your railroad. And it will be tricky to extricate them.
  • If you are running a helper district, becareful that the helix is within the range of track for which helpers are operated. Otherwise you'll need so much power on the point of your trains just to get to the helper district, you won't need the helpers. Cosmetic helpers are dangerous doing to stringlining risks.

A problem helix.

A fellow was asking about building an 18" radius helix in HO. The short answer is DON'T.

A 18" radius helix in HO isn't a good idea... You have 9.42' per lap of helix. With a 2% grade that will give you about 2.3" of railhead to railhead of rise per loop. Allow .5" for thickness of roadbed and track and you have 1.8" of overhead clearance in the helix. This would be marginal for N scale and impossible for anything other than flat cars in HO.

Plus in HO scale having an entire train on an 18" curve will create an apparent 2% or so of extra drag (due to the wheel sets not having a differential so either the inner, outer, or both wheel sets will be slipping/scrubbing) for an apparent 4% grade! Yikes...

If you take the helix grade to 3% then you will have a railhead-railhead distance of 3.39" and a clearance of 2.89". With an apparant grade of 5%. Ugh. You either need to keep train lengths very short or use lots of engines (and risk stringlining).

So I'd strongly recommend againt an 18" radius helix in HO. In fact I'd suggest that an HO helix should be more like a 30" radius and a 36" or wider radius would be better!
  • How far apart are your decks and how large a radius can you use in your helix?

This page and images copyright © 2007,2008 by Charlie Comstock