[MOC] Puzzle Shunter

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I recently got a used set 60051-1 High Speed Passenger Train, with some extra tracks - for a total of 30 each curve, straight, and flex pieces, plus 3 switches.

It's the first time I've had a motorized train.  After running it in loops for a bit, and thinking about what I actually want to operate, I decided to build a small switching yard, using the classic Inglenook puzzle layout to make sure it's got enough interest.  To fit the space and tracks I have, I aimed for cars built on 6x16 plates, acquired more wheelsets and couplers, and went for it.  I found some MOCs on Rebrickable, tweaked them a bit to fit the dimensions, and then couldn't find a shunter that met all my criteria at once.  So I designed my own.


Due to where I built it, I would have wanted two left-hand switches, but I compromised on having the closer one switched from the far side of the track.  I might change that later.  The whole thing is built as a siding from a loop track off the right side of the photo; a level crossing marks the end of the puzzle.  The tracks fit 3, 3, and 5 cars.

Left to right, starting on the top row:

MOC-5453 Stake Wagon, 4544-1 Car Transport Wagon, MOC-61497 Tank CarMOC-61570 Cargo Wagon

MOC-61575 Small GondolaMOC-68296 Barrel Tank Car, MOC-61570 again, MOC-61639 Hopper Car, and the shunter.

Six of the cars are built to the identical 16 stud baseplate, the two tank cars and shunter are 17 studs long because of practical piece limitations.  One side effect of this is that the middle shortest siding can't fit both tank cars at once.  Fortunately due to symmetry in the puzzle, that's never required, it's just an additional thing to keep in mind.  The other two sidings have about an extra third in space past the minimum 3 and 5 cars, so they can fit the slightly longer cars.

Actually playing the puzzle is a matter of pulling 5 random plates out of the gondola, and building a train with those cars in order. (medium azure, yellow, white, lime, blue, dark orange, reddish brown, and red)


One of my other goals was to not have to touch the train while operating the puzzle.  Other than when the magnetic couplers refuse to stick to each other anyway.  So I built a decoupler that inserts a pair of bionicle teeth between the buffers, and pushes them away from the locomotive.


Raising the liftarm lifts the teeth into place.


Pushing down rotates the left tooth, separating the magnets enough for the shunter to pull away.


On straight track, this works well - the decoupler on the right, marked with yellow, can decouple full strength magnets.  But there are a couple problems with this.  First, a couple of the flat cars are so light that they can actually be derailed if they're decoupled too violently.  Second, as soon as the cars are in a weird position because of the squiggly part of the switch track, the buffers are too far apart for it to work.  Third, once it's transmitted through axles and u-joints, there's a lot of flex in the system, so farther away decouplers work less efficiently.

Problems are resolved by attaching 44728 bracket to the couplers - only on one end of each car, since the layout has no turnaround.  This reduces the 'full strength' by quite a bit, downside is that a bad coupling is hardly enough to pull cars on flat track.  But they do decouple consistently.  There might be better options in minifig neckwear that have a partway between compromise, but as it stands, if they're all connected properly, there's no issue pulling all 8 cars.


The shunter is not under as tight length restrictions because it's only affecting one track length (distance to the level crossing), but I was designing my own anyway for other reasons, so my targets were

  • Use parts I have.  Mostly this means Power Functions, with train or M-motor.
  • Low speed.  The decouplers have about 1 stud of wiggle room, I don't want to have to go back and forth a bunch of times trying to stop on a dime.
  • Shorter is better
  • Low hood, 2 axle design.  I want it to look like a small shunter, not a medium one that happens to be very short.
  • 4 wheel power for maximum pulling for its weight.

In the end I did reach my goals, although the build is at least a plate taller than I'd really like, if not two.  I had designs that were lower, but they either couldn't connect front and rear axles, or moved them so far apart that it didn't look prototypical anymore.  With the design I went with, the 12 tooth bevel gears fit as close underneath the battery box as possible, and there's only one layer of tiles on top of the battery box.



The motor just fits inside the doorframe, its bottom edge is sitting on the sill.  1x4x5 train doors would technically also work, but I don't have any.  So instead I'm using flags to cover the bottom half of the window.


Accessing the power switch is a matter of lifting out the jumper plate using the exhaust pipe.  I'm not playing roughly with it so I don't expect it to fall out, but it is just resting in place.


Can't get a lot more compact than that - the transmitter is sitting on the plug on the battery box, and the bottoms of the transmitter and motor are touching.  Wires are wrapped around the motor and folded into the 2x2x4 open space under the transmitter.  A downside of this is that the remote works more consistently from in front of the shunter than behind it.  The front coupler is touching the battery box, so no room to make it shorter there.


The motor goes down through the coupler at the cab end, then a series of gears forward through what should be a pair of 4x6 technic frames.  So far it's been performing exactly as I need, has no trouble pulling all 8 cars, and the puzzle never requests more than 5.

As for what it still needs, aside from parts I don't own, ie. a different doorframe (dark bluish gray?) and consistency on the roof (black? dbg?), I'm debating swapping tan for dark bluish gray.  I feel it's a little too busy in colours, having black-dbg-tan-lbg-red, and the red is bright enough that it won't start looking drab if more of it is grey. I'm not sure about colour theory, but the tan might also be making it look taller?  I might also try putting the windows a stud lower.


I don't have a good camera, or tripod, but here's a short video of the shunter and decoupler in use.  Lower than 1:1 gearing might have been nice, but this slow speed is adequate.


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I don't think you could get any more efficient in your use of space for the locomotive.


As for the decouplers, those are brilliant. Right now someone has to view the video to see how they work, I'd suggest you take a few photos of them in action (without trains) so that someone can see the steps they take.

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On 2/18/2024 at 12:36 PM, zephyr1934 said:

I'd suggest you take a few photos of them in action (without trains) so that someone can see the steps they take.

Oh, that's a good point, I only showed the operational aspect, not a focus on the mechanism.


87408 "toggle joint" is the only unusual piece, the rest's mostly plates and Technic bricks. In the 'open' position it lies flat on the 2x10 plate to the right beneath it, and in the 'active' position it's stopped by the railroad ties. Also using 11458 1x2 with pin hole, for pinholes aligned with studs, 1x1 or 1x2 technic bricks with 2 holes work, but are taller.  Altogether this is 4 plates below the train track, 3 for the structure and a 4th so the bevel gears aren't on the ground.


The elastic band is holding the teeth together, so turning the handle 90 degrees lifts it up first.  The yellow tile between the ties is to indicate alignment, matches the points of the teeth pretty well.


Moving the handle farther stretches the elastic and spreads the teeth apart.  In design, I have a bushing on the end of the axle for the elastic, but this is an older one that's quite stiff, and going straight onto the axle stretches it less.

In practice the shunter's motor can't be back-driven, so it stays still, and the other cars move, but the stationary tooth also holds a buffer to stop it from moving.



Two ways to modify this for symmetry purposes -

  1. Move the black 12t gear to make the handle rotate in the other direction.
  2. Move the tan 12t gear and stationary tooth, for it to spread in the other direction.

And of course the whole thing can be built flipped symmetrically if it's more convenient to have the handle on the left of the decoupler.  Or the handle attached from the other side of the tracks.

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Nicely done with the puzzle. Your shunter is about as small as one can get with a PF setup.

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