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A Portable Railway Overview

My portable railway came about mainly because of the confluence of two drivers: the loss of my Garden Railway, due to a house move; and the need for our small group of live-steamers to have a railway for public show use. Such a railway imposes a lot of logistical, hard-work, and operational, difficulties on its users; none of which has anything to do with running trains. These three difficulties are (non-exhaustively!) exemplified by: storage and transportation, setup and teardown, and track levelling and reliable point switching. Clearly, these issues had to be addressed by the design to the extent possible. Also, I wanted to produce a flexible railway if at all possible, one that could be assembled with different track configurations (think H0 train sets); so the railway module design had to support extensibility by being standardized in a simple way.

When I started thinking and designing I thought that it was more likely than not that I should find it infeasible actually to make something: the railway would be time and money expensive, storage and transportation always are problematic, finding a site for regular use might be difficult, and so on. This pessimism was somewhat liberating! I could take my time. Also, I could design something that might be idealistic, but fun to do. As it has worked-out, hardware now exists - I was wrong in my pessimism; although it remains to be seen if building the railway is sensible or not.

My thought processes during design are largely lost in the mists of time; but I followed the fun path mentioned above and did what appealed to me. The railway is not up and running, as can be seen in the image, which shows half a dozen modules put together arbitrarily (think H0 train sets). Clearly, I have not yet got to track-laying.

The initial construction project comprises twenty-four of these modules; six are shown above, but all of the twenty-four are at the same stage of completion. These modules form a very ordinary oval that fits in a rectangle 32ft[11m] by 44ft[14m], with 15ft[4.6m] radius curves, and an oval centreline length of 115ft[35m]. Junctions for steamup bays, not yet made, also can be seen in the diagram. What follows is a high level description of how I arrived at this point.

Abstract Design
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Engineering and software coding feature heavily in my work background; so producing track design software was an early and natural activity for me. Non-technical people may find it frightening, but things such as the abstraction of using three two-dimensional vectors to model a module, together with appropriate vector arithmetic, etc., has been highly successful. Apart from the track design aspect - see the diagram above - the mathematical and computer models have provided a cohesive foundation for everything else, including module structural frame design, full-size paper templates for module manufacture, and module hardware size verification.

Concepts that come out of the abstraction are not always conventional. For example, curves are defined by their number of modules per circle and their arc length, and not by their radius; the latter being an arithmetic result of the former two. Thus, in the track diagram, the main curves are 16mpc, i.e. sixteen modules per circle, so they each have a turn of 2212 degrees. And the arc length is specified to minimize the expense of rail/track wastage. Track and rail is sold in the USA in lengths of 72 inches (six feet, [1.83m]). In the railway now under construction, the curved modules' arc length is set to 69 inches; resulting in the longest rail on a double track, 16mpc, gauge 3, module being 71.45 inches. The tracks centreline spacing is 10 inches [254mm]; and the outer track, module, and inner track radii are 15.06[4.59], 14.64[4.46], and 14.23[4.34] feet[m]

Part of the "fun thing to do" of the mathematical and computer modelling was to implement transition curves in the form of Euler spirals. The Euler spiral is the modern transition of choice for roller coasters, roads, and railways, and I wanted to understand it. Thus, in the diagram, the four modules adjacent to the two straights are transitions to and from constant curvature modules.

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The first hardware task to be considered was the interface system, i.e., how the modules are connected together. As an acknowledgment, some encouragement and inspiration for my solution ideas came from reading about the Connect and Module systems developed by the Gauge 3 Society; although, for various reasons, my design is quite different. Note that the Society writings about the systems mentioned are not publically viewable.

The basic interface plate comprises an aluminium angle with a locating pin and locating hole. Two interface plates are held together by commercial over-centre latches mounted on the bottom side of the angle. These two things, pins and latches, provide a precise, quick, and secure, connection. However, the design does produce a rolling torque about the bottom edge of the aluminium angle, generated by the latches, and which must be resisted by the module that hosts the plate.

An interface plate can be attached to any module, there is no requirement about how the plate is used. However, a normal expectation is that the centreline of a track be aligned with (midway between?) a locating pin/hole pair. Normally, a basic module has two plates, one at each end; similarly, a module which is a simple single track railway junction will have three interfaces. More complex interface plates come into being as the result of the subject railway design; for example, a two-track railway usually has two interfaces, one for each track, although probably they will be implemented in one piece of metal. The key thing to grasp is that it is the module design that specifies the number and placement of interfaces; but all the interfaces are identical. The modules that I have constructed are standardized straights, curves, and straightforward junctions, so that railway re-configuration enables straightforward module re-use (think H0 train sets).

This image shows the features described above. The modules are laying upside-down showing a single track module latched to a double track module. The hole in the centre of the double track interface is a "service hole", it has no function, I put it there in case a hole in the module interface plate centre ever is needed. The next hole is the second track locating hole, and on the other side of the unused latch is the second track locating pin. The hole between these locators is offset from the centre of the (single) track interface, and also is a service hole; it is used by instrumentation that measures a finished module.

Structural Ladder Frame
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I decided to to use a ladder frame with a single stressed skin for my modules. The sides, being key structural members, are aluminium bar. However, the ladder ribs are wood. Aluminium ribs would be better from some points of view, for example, if a module were destined to be installed outside in the weather. However, module manufacture would be more complex with aluminium, probably requiring welding or pop-rivets, etc., especially when leg connections (next section) are brought into the picture. The stressed skin is, of course, the track deck. This, too, is wood, being Baltic Birch plywood. Although, if I had decided on aluminium ribs, probably I should have chosen some form of aluminium sandwich board for the deck. An all-aluminium module is very appealing for permanent outside use.

Modules are glued together using high-quality epoxy. Mechanical fasteners and welding were contenders; but, once it was decided to use wood in the construction, using glue appeared to be the simplest course. I solder, but do not weld, so it would have been necessary to buy equipment and learn how to weld: too hard at present. Mechanical fasteners were judged to involve too much accurate drilling and possibly thread tapping, again, too hard.

A second skin on the bottom of the ribs would make a module very stiff. However, first indications are that a single stressed skin is perfect for the application. The current modules are strong, light, and dimensionally stable; but, also, are able to twist slightly. This characteristic allows a little track super-elevation simply by using shorter legs on the inside of a curve.

A module assembly jig was made during prototype development. In use, a full-size paper template is installed in the jig. The jig has interface plates that can be moved to accomodate module size and interface plate alignments as shown on the template. Component pieces are prepared using the template; for example, aluminium sides need to be pre-curved if the module requires it. A module is glued together in the jig, and can be removed after a twenty-four hour glue cure.

The image shows a two-track, curved module, ladder frame being constructed. The four separate interface plates are for subsequent modules; they show the wood blocks used to glue the plates to the sides.

Then the ladder frame is glued to the plywood deck.

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In the early designs the legs were the usual folding-table steel-tube-frame hinged-at-the-top arrangement. But after some time I decided to separate modules and legs entirely.

The current design puts sockets in the module frame wood ribs. The sockets are aluminium tubing with steel washers at the bottom to protect the wood. Support for a module is left to be done in whatever way is appropriate for the operation site, but presumably using the sockets. Also shown in this image is a keeper, which is the manufacturer's term for an anchor for an over-centre latch.

Usually, legs are required. The current legs are assemblies of aluminium telescoping tubing with a collet lock for length adjustment. Each leg has a rubber foot, with a steel washer insert to spread the load. At the top end is a magnet glued onto a non-magnetic support washer glued into the upper tube. Most modules have six sockets, and a leg can be put into any of these. The leg magnet binds to the steel washer in the socket, holding the leg in place. The magnet is there solely for setup purposes: the magnet is strong enough to hold the leg in place during module maneuvering, but weak enough to release comfortably when the leg is pulled out. In use, the weight of modules and trains is supported by the steel washers, the aluminium tubing, the collet lock, and the rubber foot. The legs allow a track deck height from 24 inches [0.6m], which is kid's viewing height, to 42 inches [1.1m], which is old geezer's train-running height. The adjustable legs also enable setup on rough or sloping ground. The legs are stored in boxes separately from the modules, which aids in storage and transportation. One of these boxes is just visible under the junction module in the image below.

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The description above gives an overview and the status of this portable railway project. The project is continuing on the two areas of work required for project completion: racks for storage and transportation, and track-laying. What follows are a few items that may be of interest.
  • The little green bugs on the tops of the module decks are spirit levels that indicate both along and across the module. It is hoped that these will be a lot less obtrusive once the tracks and other railway stuff are in place.
  • Assembly of the oval railway by two people takes about one minute per module, and there are twenty-four modules. However, this time does not include fetching, carrying, etc. Assembly can be done by one person, but two is vastly better. What is needed is a strong person and an agile person, agile at the end to be connected, strong at the free end of the new module. Agile latches the new module to the old assembly; then strong holds the module exactly level and becomes an unmovable rock, while agile plugs in the required legs and tightens the collet locks; they both let go and check the installation, maybe adjust it - blaming each other for the discrepancy; and then they move on to the next module.
  • All modules are self-standing with three legs; however, the single track modules are a bit wobbly by themselves. This condition changes as the railway is assembled; the railway acquires rigidity and heft as each module is added. It remains to be seen if leg stiffening is required at key points.
  • Each module weighs less than 15lbf [6.8kgf]. Track will add to this.
  • Each leg weighs 1.25lbf [0.6kgf]
  • Storage racks (underway) are 76inches[1.9m] by 24inches[0.6m], and allow 4inches[10cm] height for each module. All modules fit into this envelope with space to spare for track.
  • All the module aluminium was primed with etching primer, this is the green paint on the interface plates; the wood was primed with exterior water-based primer. The brown is two coats of water-based porch paint.
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last-modification-date: 25 Jul 2017