This article describes the planning, design, and construction of a helix for the Pocahontas Division, an N scale model railroad of the Norfolk and Western's most famous and picturesque division, set in July of 1957.
Both full and empty coal trains arrive and depart Williamson from both directions, making it possible to see full coal trains passing each other headed in opposite directions. All loaded coal hoppers into Williamson and all empty coal hoppers out are to and from the mines. All loaded coal hoppers out of Williamson and all empty coal hoppers in are to and from the N&W's customers. N&W's customers and the mines served lie both to the east and west of Williamson, and the huge volume of coal traffic make Williamson yard the largest marshalling yard in the world.
For eastbound trains, Williamson marks the beginning of the grades required to ascend the Allegheny Plateau and cross the Allegheny Mountains. It was at Williamson that the Class A locomotives which pulled the coal drags through Ohio were exchanged for Y6b's, with their incredible tractive effort exceeding 150,000 pounds. Eastbound from Williamson, the ruling grade is 0.6% for 74 miles without a single downgrade, gaining a total of 300 feet in elevation. East of Welch, West Virginia, the ruling grade stiffens to a nearly continuous 1.1% for ten miles, and beyond Northfork the grade runs at a steady 1.4% for the last seven and a half miles to the Elkhorn Tunnel, gaining 1300 feet of elevation in only 17.5 miles. Helper service would normally be added (150,000 pounds tractive effort behind the caboose!) just west of Welch and continue on through the Elkhorn Tunnel into the summit at Bluefield, Virginia, thirty miles east of Welch.
To model this division and do it justice, several conditions have to be met:
- Williamson yard has to be large. The prototype yard is incredible, filling the wide valley of the Tug Fork for miles. To get any feel of the prototype into the model, the yard must be large by modeling standards.
- The vertical separation of Williamson and the Elkhorn Tunnel should be large. There should be a clear sense of Williamson as the bottom of the hill, and Elkhorn Tunnel as the top.
- There has to be a lot of staging. In addition to the dozens of coal trains, full and empty, which arrive and depart Williamson each day, both high speed and local passenger service run through the division, but are not originated or terminated there.
- There has to be an off-stage means to get trains from the top of the layout, the Elkhorn Tunnel, back to the bottom, and vice versa, during an operating session, in order to meet the traffic requirements. While loaded and empty hoppers can go into or out of Williamson from either direction, the mines east of Williamson are not as far east as the Elkhorn Tunnel, making all traffic through the Elkhorn Tunnel loaded eastbound, empty westbound. This makes it impossible to turn trains at the top and run them back without obviously running coal back into the coal fields.
- There has to be a way to provide mine traffic into and out of the layout over branch lines to coal mines and destinations which are off-stage. These branches connect into the main at different points, and therefore at different elevations.
- The railroad was planned as a double deck railroad. This allowed enough space to be allocated to Williamson yard to get the feel of the prototype while leaving enough room for the rest of the division. The two decks form a two lap helix within the main layout room, giving a main line which climbs continuously from Williamson yard at the bottom to the Elkhorn Tunnel at the top. The two laps also allow enough running room to give a substantial vertical separation from Williamson to the Elkhorn Tunnel.
- A helix in the next room would be used to connect the top of the layout to the bottom, allowing trains to run through the layout always in the same direction, loaded hoppers into the Elkhorn Tunnel, empties out.
- The helix would be double track to meet the traffic requirements without being a bottleneck, especially due to its required length.
- The helix would also allow a train to return into the layout from the direction it left, allowing out-and-back passenger service to be modeled correctly.
- The helix would not violate minimum curvature for hidden track or exceed the ruling grade for the layout (except for the helper district). The helix was not to be a helper district on its own, nor would special coupler lengths or equipment clearances be required.
- The helix would provide staging yard leads from the laps of the helix, allowing multiple staging yards to be stacked above each other.
- The staging yards at each level would also connect into the branch lines for that elevation, running into the layout room and under the scenery to emerge at the proper point and elevation to model the traffic to and from off-stage mines and destinations.
(40") / X = 1.5 / 100 ==> X = 2667" or 222'3" long.
Allowing for thickness of plywood, roadbed, track, vertical clearance for any type of equipment, and some overhead for power feed wires, switch motor linkages, and five-finger rescue of stranded equipment, a three inch per lap rise was considered a minimum. As a minimum the helix then needed to be:
(40") / (3") = 13 + laps long with (222'3") / 13 = 17'1" per lap.
Staging yards of five to six tracks each would meet the staging requirements if six such yards could be accommodated. Also, six inches was considered a minimum vertical separation -- after all, five finger intervention here is the rule, not the exception. A staging yard lead off of every other level of the helix would meet both requirements.
The double track helix would be counterclockwise as viewed from the top, to preserve right-hand running and put the climbing track on the outside where curvature would be less -- i.e., the larger radius turns would be on the climb, lessening the risk of pulling the whole train off into the abyss.
Access to the staging tracks would be only from the climbing track, there not being enough vertical separation between laps to extricate yard leads from the inner descending track. Exit from the staging tracks would be straight out, engine first, uphill. Entrance to the staging tracks would be gained by climbing the helix past the trailing point yard lead turnout, and then backing the train downhill into the yard. Staging yard lead turnouts would be laid so that the undeviated track would go into the yard, lessening the risk of the backing procedure. Staging yard tracks would be at a slight downgrade, about .5%, to assist the backing procedure and keep loose trains from rolling out into the helix.
The requirement to allow turning a train and sending it back into the layout on the reverse path was the tough one. The access to the staging tracks being only from the climbing track of the helix also required being able to run a train down the helix to the bottom and then back up to the staging track desired, and from the staging track up the helix to the top and then back down to the bottom to enter the layout. Otherwise the staging would only serve traffic which was climbing on the helix and descending on the layout.
The solution seemed to be a reverse loop on both the top and bottom of the helix -- but how? A separate reverse loop would require an additional area larger than the helix itself. Another solution was to put the reverse loops within the area of the helix in sort of a ying/yang approach (Figure 1), but that made the radius of the reverse loops half that of the helix itself and included a nasty S turn.
The solution was to stop thinking about a circular helix. Consider a circular helix to be two half circles, and spread the two halves, joining the halves with straight lines. The reverse loop is now a teardrop shape, the minimum radius of the reverse loop is the same as that of the helix, and the nasty S turn is gone. A little thought will convince you that the minimum separation of the two half circles to get rid of the S turn is just a little over twice the radius of the half circles; at exactly twice the radius, the reverse loop has achieved the same minimum radius as the half circles, but the S turn remains (Figure 2).
The actual dimensions chosen for the helix for the Pocahontas Division are as follows:
Radius of end half circles: 14" inner descending track 15 1/2" outer ascending track Separation of half circles: 63 1/4" Number of laps: 12 1/2 Lap vertical separation: 3 1/4" Lap length: 214 1/2" inner descending track 224" outer ascending track Grade in percent: 1.52% inner descending track 1.45% outer ascending track
This has resulted in a very nice form factor for a helix. The entire construction is 36" wide and 100" long, and fits very nicely along the outside of the layout room wall. A circular helix of the same length per lap would exceed six feet in diameter. The helix plan is shown in Figure 3 (first lap), Figure 4 (even laps 2, 4, 6, 8, 10, and 12), and Figure 5 (odd laps 3, 5, 7, 9, and 11). The top partial lap is the reverse image of lap 1 without the staging track lead.
The oval helix form factor is probably more suitable for N scale than any other scale, due to the fact that the minimum radius of 14" used here is considered a medium radius turn in N scale. In HO scale, a 28" end radius with an 84" separation of the half circles would result in a helix module approximately 5' by 12', and a lap length of 344". A 1.5% grade would give a vertical lap separation of a little over 5". This may not be too bad a unit for a club layout.
The frame is a simple box of two-by-fours measuring 96" by 32 1/2". A single cross brace provides sufficient rigidity for the two long side members and allows room later to stand up in either end of the unit. Diagonal corner braces aid rigidity. One two-by-two leg is fastened to each corner.
The subroadbed is supported by the slotted riser technique. 26 risers 46" long are required, eighteen outside the subroadbed and eight inside the subroadbed, and they are almost identical. The risers are manufactured by clamping two-by-twos together with pony clamps and repeatedly routing the slots with a 1/2" router bit across all of them at once. Three-quarter inch deep slots were routed in the risers, taking three 1/4" cutting passes from the router for each of the 13 slots in the risers.
The subroadbed itself is cut from 1/2" plywood into four types of pieces:
- straight sections: twenty-three required. 63 1/4" x 5".
- curved sections: forty-two required. 12 1/4" inside radius, 17 1/4" outside radius.
- curved section with staging yard lead: six required.
- teardrop sections: two required, one the mirror reverse of the other.
The straight sections were cut to size on a table saw. The curved sections are produced by making one piece and then tracing it on plywood the required number of times to create the rest. The first curved section with staging yard lead is traced from the master curved piece and the staging yard lead drawn in by hand; the other five are traces of this one. The teardrops are traced using one straight section and four curved sections; the diagonals are drawn in tangent to both end curves with a large straightedge. For the second teardrop, trace the first, but upside down to get the mirror reverse of the first. The shapes and dimensions of the parts are shown in Figure 6.
When cutting out the traced parts, center the saber saw blade on the pencil line. This will yield a part the same size as the one traced. To add rigidity to the diagonal of the teardrops, a one-by-two is glued and screwed along the bottom center of the diagonal as a stiffener (Figure 6). Glue the one inch edge to the bottom of the subroadbed for the maximum rigidity, and make sure the stiffener is not so long on the upper teardrop that it is in the way of the trains on the lap below.
All parts are painted before assembly. A single coat of latex paint will help stabilize the wood against humidity variation and prevent heaving and buckling of the track. It will also prevent splinters in the construction and operation staff. Do not paint the inside of the routed slots in the slotted risers: the 1/2" plywood will fit into the 1/2" router slots loosely before painting, tightly after painting the plywood, and not at all after painting the slots.
The ten slotted risers on the outside of the two ends are fastened to the frame first with three inch drywall screws. The two teardrop sections, one at the top and the other at the bottom, are installed next. The remaining eight outside risers are then installed. The helix is now constructed from the bottom up by gently and delicately sledging each subroadbed section into position with a plastic mallet. Make sure that the risers remain plumb; violators are similarly persuaded to mend their ways with the mallet. Riser and subroadbed locations are shown in Figure 7.
The order for installing the subroadbed pieces is as follows:
lower teardrop curve \ curve with staging yard lead \ straight \ curve \ curve \ straight \ Repeat this sequence 6 times; the sixth curve / time do not put in the last straight. curve / straight / curve / curve / straight / upper teardrop (instead of the last straight)
Lay roadbed and track before you cover up any subroadbed with the next lap of the helix; the best bet is to install three-quarters of a lap of subroadbed, install roadbed and track to within a foot or two of the end, and then repeat. Be sure to install your power feed wires to the track at this point as well; doing it later is not fun.
The last half lap under the upper tear drop must be installed in the three inch gap between laps; there simply isn't any way to get the tear drop into place once all the outside risers are on, and there's no way to install any subroadbed until they are. The AMI instant roadbed and Peco flextrack used on the Pocahontas Division are installed using rubber cement, which is much easier to work with than track nails. This method provides an extremely strong continuous bond, preventing heaving and buckling of the track with humidity variation. The roadbed will still allow a certain amount of expansion and contraction without losing the bond, and construction mistakes can still be ripped reasonably easily.
Once the subroadbed, roadbed, and track are completed the inner risers are installed. Six of these are required to keep the edges where the subroadbed sections meet from displacing vertically from each other and heaving the track. The other two prevent sag on the inside edge of the long straight subroadbed sections. These risers are not attached to the frame at all, but are simply bolted through to their outside companion risers with threaded rod. The threaded rods should be as close as possible to the underside of the subroadbed so as not to diminish vertical clearances.
The Pocahontas Division standard is that both mains are wired in the same sense, so that eastbound is the same reversing switch position on both mains. This allows trains to cross mains and use passing tracks without reversing block problems. The only reversing blocks on the layout are those in the helix. An additional reversing switch will be installed on one throttle for one of the main lines on the helix, which allows the sense of the wiring for the two main lines to be reversed. The helix can then be used as a continuous loop test and run-in track for new locomotives without throwing reversing switches at the top and bottom of the helix.
One last note: I am not a fast modeler. While Williamson yard is completed, and the helix is almost finished, all of the staging yards, branch leads, and the layout itself are not. Please do not ask me how long I have been working on the Pocahontas Division, or when it will be 'done'! For me, getting there is half the fun!