Pocatello Model Railroad and Historical Society, PMRHS. Stand alone, walk-thru modules for easy access.

10/10/02

We've all heard the nursery rhyme- For want of a nail-- a kingdom was lost. Well, it started out this way. Operation potential was the pits- Both big major yards were misdesigned and usable only for storage or holding. They needed major work, especially drill tracks. The redesign created a nasty obstacle. Access from one end to the other was wasting too much shoe leather already and this would make it untenable. We needed another walk-thru module but it could not be a lift bridge- the bridge had to drop out of the way. This was our "nail"

The photos give you some idea of what we encountered and what was done to overcome the problem(s). All photos are taken with a digital camera by our club photographer Conn Housley

just click on the thumbnail for a 640 x 480 picture.

You can see from this closeup of the track plan, the big yard has been split with the double ladder. It would sure be nice to get rid of the four foot WYE module at this busy location and to have easy access between the West and East yards. We need a walk-thru, but why not a duplicate of our existing one? In this case, operators will be using it very frequently to gain access to a double ladder tracks serving the East & West yards, while the need to run trains across it to the South Holding & Staging yards will be infrequent. Dropping the bridge down, gets it out of the way where it will be most of the time.

Here is our very successful Lift-Bridge Module that has been in heavy service since shortly after our big move, about three years ago. Being a modular club, our walk-thru modules have to be free standing and that creates some unique requirements. The first requirement, as any geologist will tell you, to build a bridge you must find stable earth to set the piers upon. Our resident anthropologist estimates the the HO Godzillas traversing the Canyon weigh as much as (300*87^3)/2000= 98,775 TONS. That can cause some serious tremors, but the canyon walls have held and the bridge drops accurately in place every time. We used the same basic techniques for the drop-leaf WYE module

THE CANYON: We built the four foot canyon-module using the proven design used in the lift-bridge module with only minor changes to make it into a drop-leaf. The framework was assembled with 2x3 lumber rather than 2x2s like the original simply because the 2x3s at Home Depot were better quality for the dollar and we really don't expect to take this heavier module to shows. A sheet of quarter-inch plywood is cut into two "U" shaped sections and other pieces for triangular bracing, and assembled into a strong but light weight structure. The design insures that Godzillas weight is transmitted toward the center of the earth without distorting the module. The six inch notches on the ends are unique to our application as the origianl WYE was only three feet long, hence this four foot module tucks a bit under the adjacent ones. We needed the four feet for structural strength.

HEIGHT ADJUSTMENT: There isn't any. Unike common modules, there is no provision for height adjustment. If the floor is uneven, we'll use a shim under a leg. The track height is accurately set to the common 40 inch standard and then adjacent modules, that Godzilla doesn't walk on, are adjusted to meet it.

DROP LEAF: With good stable canyon walls established we dismissed the Geologist and Anthropologist and turned design over to our own Engineering Department. the PMRHS Chief Engineer moonlights for a little run-down mountain railroad the Teton Short Line. where he practices lore that is occasional useful to the PMRHS. As you can see from this aerial photo, our drop-leaf is wider at the hinge end because it carries two legs of a wye, but that is only a detail. The drop-leaf is simply a sheet of 1/2" chip-board (it doesn't warp) hinged on one end with a piano hinge.

The open end is aligned with a tapered wood block sliding into a tapered wood socket as seen in these topside and underside shots. The wood faces are lubed with paraffin wax. We are concerned mainly with lateral alignment on this end because the track angle is square so we made it simple. Tests have proven that HO equipment can safely traverse a gap up to one-quarter inch if the rails are in line and one wheel of the axle is properly guarded. We don't need to get that sloppy, but it can be done. Potential sagging is of concern and if it happens we'll probably treat it with some queenposts and tension rods just like an old-time flat car. Might even do something unimaginative like screwing on a piece of 1x2.

LATCHES: The leaf is initially latched using our proven electro-magnetic technique. The electromagnet is assembled from an old transformer similar to something you'd find in a small throttle pack or a Radio Shack transformer in the 20 to 40 va range. The output voltage rating is of no concern. We only use the old 120 volt PRIMARY winding. The core pieces are disassembled and re-stacked with all the E-shaped pieces aligned in same direction. No fancy tools, A small vise, hammer and a scraper and/or small punch will be all you need. You can even mangle and discard a few of the "E" sections and it won't hurt. Power the old PRIMARY winding with about 12 Volt DC and it will attach itself tenaciously to a piece of steel (armature). The adjustable iron plate that you see is adequate but I added some thickness using the the pieces salvaged from the transformer core in an effort to outdo Godzilla- It wasn't enough-. The plate is made adjustable with the three bolts. When the magnet core and steel armature contact each other they also make a convenient electrical switch for interlocking.

Unlike our lift bridge, an operator , or Godzilla may lean a heavy elbow assisted with gravity and dislodge the latch, so we added a simple slide bolt for safety.

TRAIN PROTECTION: Both latches must be in place for the track to be powered. The circuit is made continuous to operate the safety relay via the magnetic latch and the slide bolt. Electric power is provided by a wall-wart salvaged from some toy or bought new from Radio Shack. Typically, the magnetic latch will need about 200-300 ma at 12 volts. The 12 volt relay is also available at the Shack if you can't find something in the junk box. When the safety circuit is open, the approach tracks are not only disconnected from power, a short is placed on them to insure that a multi-engine consist will be shut down.

TRACK GAPS: It's another of those basics. -stamp foot- test question-If one wheel of an axle has its flange captured between the running rail and a guard rail, the other wheel can't mistrack. For that reason, we stagger the gaps. The pictures make it clear. The rail ends are soldered to pieces of firmly mounted printed-circuit(PC) board insuring accurate, rugged and adjustable rail positioning

OPERATION: We are usually carrying something, a radio-cab, coffee cup, a piece of rolling stock and sometimes all three, so we made sure that only one hand is needed to open or close our bridge. To open it, slide the bolt free, then punch the BIG green button. The leaf will gently drop due to the screen door closer mounted underneath. To close it, just reach down and lift it until the electro-magnetic latch grabs it- then slide to bolt in place. If you still don't have enough hands, an elbow, chin or other body part(s) will do. We tried to make it easy.

COST: The track and electrical stuff was salvage. The lumber and hardware at Home Depot was under $50. We do hope you can glean an idea or two from our project.

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