Chris: I do not have any wedding cake helixes on my layout because the ease of access is SOOO much better for a bowl-shaped helix. I'm pushing sixty years old. At my current levels of flexibility and strength, it is still possible for me to comfortably duck walk under the framework that holds my lower deck at 48" elevation from the floor, and then stand up inside a bowl helix with a 26 inch diameter access area. I can easily reach 100% of the track from the moment it appears inside the bottom of the helix to where it exits the top on a tangent and thus passes beyond my reach while I'm still standing inside the helix. In a bowl-shaped helix, all track inside the helix is fully accessible from above. I can use a soldering wand or gun on either side of both the inside and the outside rails (although my view of the loop's outside rail's outer side is vertical instead of horizontal). Because I can position my eye directly above the track centerline, I am able to check alignment, see small obstructions that may be hidden from view when one is forced to look from the side instead of from above, and find the pre-drilled nail holes in the Flex-Track for when I want to adjust track. Picture a wedding cake helix located at the end of a peninsula. You can easily see all tracks on the right, left, and open end of the peninsula. The problems come from trying to access the back portion of the loops because the upper deck typically extends close to the helix to maximize the amount of shelf space available on the upper deck. As a result, each time the loops climb another tier, the amount of space available for viewing the rising tracks on the back side of the helix from directly above diminishes. There will be hardly enough space for track cleaning and absolutely insufficient space to perform any soldering, aligning of the track, or finding the Flex-track nailholes. If the two decks are separated by 14 inches and the upper deck is just 2 inches thick, then you will have a 12 inch tall window to reach into the tracks on the back half of the wedding cake helix. Most bowl-shaped helixes will be about 4 feet in diameter and many may have 6 to 8 inches of scenery surrounding them...so the total distance across the back of the wedding cake helix might be up to 5 feet (perhaps 24 to 30 inches from the aisle to the middle of the back of the helix). Factors affecting wedding cake helix accessibility include: 1. the elevation of the upper and lower decks, 2. the total width of the upper deck, 3. the thickness of the upper level shelf, 4. aisle width (which affects how close you must stand to the front edge of shelf...and ultimately how much you'll have to bend over to see the higher loops closer to the upper deck). Reaching to the upper loops at the back of the wedding cake helix will require back-breaking contortions, such as holding yourself bent over in a 12 inch tall space between the decks while you work in the darker areas close to the upper deck with your arms fully extended and absolutely no way to look at the back rail (the one on the inside of the loop). This pic shows several tracks in a type of wedding cake tier configuration as the tracks pass through a doorway. Image trying to work on the top track if there were a 4 foot wide deck an inch or so above the top track, and a 4 foot wide deck under the lowest track. Or, instead of a single track disappearing into this bowl shaped helix, picture the lowest wedding cake helix tier about where the single track disappears into the Bartonville Helix and the highest tiers up under the upper deck. You wouldn't have much working room on the upper tiers. In this picture, the blue board appears to be resting with its lower right corner on the track...but, the camera angle doesn't show too well that the board is actually about 2 inches above the track. If the blue board were to represent the underside of the upper deck as the top tier track begins its final loop onto the upper deck, you can see how little space there would be for maintenance or repair.
With the wedding cake helix couldn't you have an access hatch in the upper deck so you could potentially get into and "stand up" in it the same as a bowl helix? The room inside could be the same, it would just go from a larger dia to smaller dia as you go up. You would want bullet proof track for sure, but I would think thats the case with all styles of helix. You wouldn't want to close in between the spans so you didn't have access from the inside as in the photo's. You would also potentially have access from outside as well Would access from the middle be that much more difficult than a traditional designed 2 track helix? thanks Chris PS: I guess it would be difficult to have a 2 track sprial helix, and still have the ease of construction?
Personally I don't think that access would be the biggest factor for me if I were making a decision about this. (All else being equal, however, I would think that arranging access from inside a "bowl" would be the easiest.) I don't think it would be meaningfully more difficult to construct a 2 track spiral helix. However, the problem with a 2 track spiral helix is that the radius would have to increase/decrease twice as much with each circuit, which greatly magnifies any disadvantages that the spiral helix (bowl or wedding) has over a traditional helix. Namely, that is, it gobbles up your model railroad real estate something fierce.
Standing inside a bowl helix, you have direct access to all of the track of the helix, standing inside a wedding cake helix, you have to reach over the top loop to access any other loop of the helix. The advantage of the bowl is once you get inside, you have direct access to the whole helix, and the disadvantage is you have to duck under the lowest loop to get inside. The advantage to the wedding cake is you have direct access to most of the helix from the outside so you do not have to duck under the lowest loop, and the disadvantage is you do not have direct access to part of the loop because of the benchwork that brings the tracks into the helix.
Standing inside would be the same, but you would not be able to see the tracks because there would be a 1.5 inch overhang obstructing your view. You would have to squat down to see under the overhang, because bending over would not be possible. This would be a very uncomfortable way to go!! I closed in my Peoria Helix after my elbow inadvertently caught a loco and prompted it to take The Big Plunge. The Peoria Helix has 5 turnouts where trains turn off of the helix into staging or to different parts of the upper deck. I usually duck into this helix when I'm running trains so I can watch them through all the turnouts. Standing inside isn't absolutely necessary, just an extra precaution. Because I'm in there so much more, there is a greater chance my elbows might end up prompting something unintended to happen, so I have opted to tack up cardboard between the loops. I don't have the other 3 helixes closed because it is unnecessary (I rarely duck inside when a train is running through the other helixes, choosing instead to watch from outside). If you decide you want to install a wedding cake helix despite the access issues, you may want to consider closing the spaces between loops to make the scene a little more "finished" rather than leaving all the subroadbed and supports visible...sort of like putting a fascia board across the front of a shelf hides the frame or stringers that support the layout. Absolutely! The traditional stacked helix with 2 tracks and a 4 foot diameter would have about 3 inches between the railheads on each loop top to bottom. Standing inside the helix, you would have over 3.5 feet of space and from the inside edge of the ramps to the outside edge would only be about 3 or 4 inches. The ramp directly above would not be providing any overhang to obstruct your view of the upper loops and you would be able to move back nearly 3.5 feet to gain a better viewing angle to the outside edge of the lower loops. With a 4 foot diameter wedding cake helix, you would only have about a 27 to 28 inch diameter center space because the supports to the upper loops extend all the way from top to bottom of the helix and must be directly beneath those smaller radius upper loops. The space between railheads on the highest loops would be about 2 inches and about 3 inches on the lowest ones. If you design your supports like my bowl helix supports, then you will have a vertical support on either side of a pie piece shaped section of the helix. There will be about 8 inches between any two supports on the inside of the helix and about 18 or 19 inches between any two supports on the outside of the helix. To have access to the lowest loops from the inside of the helix, you will have reach between the vertical supports (8 inches apart from each other) and 1.5 inches under the top loop, 3 inches past the inside edge of the top loop to get to the 3rd loop, 4.5 inches past the inside edge of the top loop to get to the 4th loop, 6 inches for the 5th, 7.5 for the 6th, etc. There will be a 1.5 inch overhang that is 3 inches above and inside of the lowest loop and about 2 inches above and inside of the 2nd loop from the top. Not impossible working condititions, but certainly not pleasant or as easy as the traditional stacked helix... and far more unpleasant than the bowl shaped helix. As Ben pointed out, you will need LOTS of real estate: about 5.5 feet diamater or over 30 sq ft of floor space to rise 14 inches between decks at a 2% grade.
