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Commander Wolf posted a topic in LEGO Train TechWell, it's been more than a year since I started work on my last locomotive MOC, the China Railways QJ. Having built most of the practical engines (not too big for R40 curves) that I was visually interested in, I had to wait a bit before my interest was piqued again on the locomotive front. My inspiration came from running the QJ at most BayLUG meetings for the past year and change. The QJ isn't necessarily unreliable or difficult to set up, but it's still not very convenient: the model isn't that easy to move around or manipulate due to the size, the tender, and the number of fragile bits. The lengthy drivetrain with its fair amount of friction and torque also prevents the engine from generating smooth low-end torque. Finally, BayLUG still runs 9v at most of our shows, and the QJ can't easily be converted to run on 9v. So this is really my second locomotive to be born of functional requirements (the first was my U30B): 1. It should be easy to transport [from here to there] and move around [a layout] 2. It should be designed with robustness as a key feature 3. It should be easily convertible between PF and 9v operation 3b. The PF components should be easily removable (also helps with charging) 3a. It should run smoothly when pushed [by a 9v power car] Requirements 1 and 3 really insist that this engine be a large tank engine: for 1 I don't need to deal with a tender when transporting or moving and for 3 it needs to be big enough to fit all of the PF stuff. It actually took me quite a bit of time to zero in on the X-10-a as large tank engines are apparently pretty rare in the US and North America: it seems that even most of our branch line and shunting steam engines were tendered. But eventually I found a drawing and the work began! What I learned from the QJ is that if the weight of the loco is properly distributed, one powered (and tyred) axle is good enough to generate usable torque. From this notion I designed the chassis to have exactly that one powered axle, which I could easily remove to remove tyres and gearing for 9v operation. For the same reason, the driven axle isn't cranked either; in the QJ I would have had to remove all of the cranks and all of the wheels to access the tyres or gears. The lack of cranks on the driven axle also lets me keep the chassis articulated, which should help minimize rolling resistance for 9v operation (say compared to a 6-coupled flange-blind-flange configuration for the drivers). The drive rods are made using the half-pin in rod-track technique, and there's a bit of a hack: the connecting rods have to go around a corner due to the articulation, so the travel is longer than the usual three studs, and the connecting rods are both loosely pinned down and made of flex. As far as I can tell this arrangement doesn't add significant friction, probably because the corner is very small. The engine is designed to be powered with two M-motors, but I'm using the E-motor right now for the novelty. Unfortunately it wasn't quite possible to get as much weight as I would have liked over the driven axle: the battery box must go behind the boiler due to its height, and that really limits weight distribution options. The loose 9v motor in the front is simulating the weight of a second M-motor, and it helps bring the net weight over the driven axle to maybe 60 percent? Here you can also see how all the bits come out of the engine: almost all of the top surfaces are detachable. Whether this is convenient enough to fulfill requirement 3 remains to be seen. Construction of the body is actually very similar to that of the QJ: structural integrity is mainly provided by studs-out beams and everything else is studs up. Stickers are created at 300DPI and printed on 3M 3200-L mailing label material. This is a small detail, but it is actually one of my favorite parts, inspired by and stolen from 60052: And finally a video showing the locomotive running. The first 70 seconds is PF running and the last 20 seconds is 9v running. For PF running I'm using the AAA battery box with AAA Eneloops and the aforementioned E-motor. The E-motor is actually pretty neat: it has a wider dynamic range than the other PF motors and it is quite quiet as well. Sadly it is a little bit underpowered as well; I'm geared down 3:5 and you can still see it struggle a little in the corners during the PF segment. The 9v segment is a bit hazy, but we ran out of sunlight because DST. The engine is actually smoother than I would have guessed in the unpowered configuration: you can see how it basically doesn't lose *any* speed in the turns, and the regulator is only turned up to notch 3. Alright, I think that's all the commentary I have on this. There is as usual a full gallery if it ever gets moderated. There's a bunch of build and reference pics there that I didn't show. EDIT: Instructions for this model are now for sale on Rebrickable: https://rebrickable.com/mocs/MOC-61355/NonsenseWars/148-canadian-national-x-10-a-powered-up