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My recently produced model above, happens to be custom run from Berwyn’s Toys & Trains but is mechanically identical to other recent MTH Premier Mikados.  It had a problem derailing its front drivers at a perfectly level Ross O72 switch, on its curved leg which is in the middle of an O72 180 degree curve.  No dips/humps or anything that would account for it in the track.  It also had occasional pilot truck derailments, so I did not try softening that spring as a way to increase front driver weight; actually had to shim the pilot truck spring with a single flat washer to cure that.

Diagnosis revealed a few things that contribute to driver derailments: Center of gravity is behind the third driver axle, and the blind drivers have equal diameter with flanged so all drivers ride on the rails.  The 8-coupled driver wheelbase is long, making a sharper attack angle for the front drivers against the outside rail of a curve.  And the short driver diameter places the pull of the drawbar about 3/8” (guesstimate) higher than the driver axles, so pulling hard on a train places a lifting leverage at the fulcrum of the rear drive axle with its traction tires, taking additional weight off the front drivers.  On any hump, all of that places all driver weight on the 3rd and 4th driver sets, with combined effects taking the front drivers off the railhead.  

Lou, the MTH repair person for Berwyn’s, confirms that a few others have had similar problems on their layouts, unique to each, hard to duplicate, and hard to pin down why more people do not have the problem with these Mikados.  Unweighted, or almost unweighted even on level track, worse when pulling, with the long driver wheelbase and relatively sharp “attack angle” of those front driver flanges against the outside rail of a relatively tight curve (even O72, at least on a switch, in my case), the outside front driver climbs over the railhead often enough to not be acceptable behavior.

I don’t use smoke.  So, I separated boiler from frame and removed smokebox front, and managed to wiggle the smoke unit out by first removing screws attaching it to its bracket (accessed screw heads through open front of smokebox) and wiggling bracket out of bottom of boiler past the smoke unit body, and then the smoke unit.  Electrical connectors for cable between boiler and frame, and the front-most 4-wire connector on the board in the boiler, were disconnected to make room to get smoke unit out past the front of that board.  Two connectors for the smoke unit were taped over.

I had a 16 oz fishing weight of the drop type with a loop in its top that is slightly smaller than the inside diameter of the boiler.  Cut to length for the available room it made a 12 oz slug giving a net increase of 10 oz after removing the 2 oz smoke unit.  Using threaded hole that was smoke unit bracket attachment point as a clearance hole, fixed weight in place with a 4-40 machine screw threaded into the lead.  This moved the COG to in front of the 3rd driver axle, not quite centered but definitely placing weight on the middle two axles now.

After shimming the pilot truck spring with a single flat washer, the loco can now pull a 7 car train of Atlas and MTH 60 foot heavyweight passenger cars, through curves and that O72 switch, which is after a long O72 S-curve so it’s pulling hard on the drawbar (unweighting front drivers a bit in that process).  So far in multiple circuits of the mainline, no more derailments.  Lou at Berwyn’s was not surprised to hear of the problem, and might have taken the same approach if softening the pilot spring had not solved it (not a good choice for my loco, as noted above.). The long driver wheelbase (attack angle issue), all drivers riding on the rail unlike many other blind driver locomotives, small diameter drivers giving drawbar leverage when pulling, and rear-heavy weight distribution have combined to cause trouble for a few folks like me.

Will be interested to learn about the experience of others with these Mikados.


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@rplst8 posted:

What about increasing the spring tension on the trailing truck? Probably not as permanent a solution, but less invasive.

Interestingly, here’s another thread with a similar issue…

Thanks for the link to that thread.  Seems to be same issue.  I didn’t mention that I had checked gauge of drivers and run on rollers to watch for any bind or sloppiness in rods/valve gear etc, and found none, after an earlier conversation with Tim at Berwyn’s.  I had also smoothed and shimmed the exit of one superelevated curve that ends in hidden track where I had missed a “hump” in the outside rail, solving derailment there but leaving the problem with the mid-curve O72 switch where trackwork was perfectly flat and the driver had to be purely climbing the rail without any outside “help.”  I didn’t pursue other more remote possibilities because the center of gravity issue was so prominent.  

Some of the theories that involve wear don’t apply since this engine is new out of the box.  I just wanted to fix the problem and move on to other priorities.  Since I don’t like to use smoke, the decision was easy for me.

The idea of increasing spring tension for the trailing truck could have merit.  Given the degree of the COG problem and limited leverage with the rear drivers as fulcrum, I’m skeptical whether that would move enough weight to the front drivers to solve this, but it will be interesting to see if someone has done that and solved their individual problem.  As Lou at Berwyn’s pointed out and the other thread reinforces, this seems to be a quirky problem that shows up rarely enough to make it ideal for speculation.  Stories of actual success from folks other than myself, maybe with other approaches, will be interesting to learn about.  If someone has fixed it without sacrificing the smoke unit, that would be valuable to lots of folks.

Yeah I wonder if the casting of the boiler or frame isn’t done properly if maybe the rear is heavier than the front?

I wonder if it’s an occasional problem that only a few have, or if all of the units have a poor CoG and only a few folks pull heavy enough consists to notice.

As far as solving it without removing the smoke unit, I’ve always been a fan of lead sheeting. You can tuck that stuff n a lot of places to add weight. It’s good you got the amount of weight you did in there, but I wonder if all 10 ounces was needed.

I’ve had similar problems with HO locomotives but never O-scale ones. Something I will keep in mind.

No photos available pre-fixing.  Front drivers simply set themselves off, to the left in my case, and kept rolling till a short stopped the action and triggered shut-down.  Except at that switch, where the derailment was immediately severe with instant stop.  Pilot truck was sporadic and I didn't wait long to add the washer shim to fix that.  Hard to know about some of these thoughts; folks more knowledgeable about production variations (castings for instance as mentioned above) might have thoughts.  For me, most interested in finding others who have had the problem, especially others who have found their own successful fix.  A matter of curiosity for me at this point.  Plan to pull some long freight drags as the loco is intended to do here, and we'll see if the fix is bullet-proof or not.  Likely, the weight transfer off of the traction tires will have reduced maximum pulling power in a straight line, in return for staying on the rails.  Will report

What equipment did you use to determine this is a center of gravity problem?……I have 6 of these Mikes, and have zero issues, I’ve swapped MTH Mike chassis into brass boilers, again, with zero issues,…….you say it’s leaving the rails on O72 switches, but how about the rest of your track??…..usually, a problem with a locomotive follows the locomotive….granted, a switch can exaggerate a problem…….if it’s only leaving the rails in those curved portions of a switch or switches, I’d be looking at the frogs, etc, to see if I see a wheel dipping in the gaps, naturally, if you’ve added weight, and it solved your issue, then problem solved,….also, you could’ve left the smoke unit in there if you so desired, the smoke box door opens on these, and you can add weights in the gap between the smoke unit and the door ( the smoke box area ) …..remember, less weight overhanging an end, is just as effective as a lot of weight over a driver, or the suspected trouble area…..but if smoke ain’t your thing, then you do what you like……if weight is needed on these engines on a particular brand of track, opening the smoke box door and packing a coin weight, or flat faced  tire weights in the smoke box door area should do the trick,……


Last edited by harmonyards

Just out of curiosity, have you compared the size of your driver flanges with the flanges on an older generation MTH Premier Mikado? 

The reason I ask is that the front drivers my 20-3703-1 PS3 0-8-0 derailed frequently on my tubular switches, but my 20-3119-1 PS2 0-8-0 never has problems.  My initial thought was that the different drawbar / tether style was causing the issue.  The problem turned out to be that the flanges on the 3703 are smaller than the 3119 flanges.   Adding extra weight to the front of the 3703 has helped, but it's still not a perfect solution.

Sorry, can't provide comparison with an older MTH Mikado for flanges.  Would not surprise me if that was a factor.  These are pretty deep flanges, and their edges strike me as kind of sharp, so grabbing and climbing a rail may be more likely than for a different edition of the same chassis.  Other than GRJ with his 2014 thread, I hope there are more case reports coming (with solutions would be ideal!)  No one I know of locally who could provide a loco to compare.

It climbs the point rail on that O72 switch, but I can't feel or see any roughness or defects in that rail to explain it.  If other locos do it, we'll for sure get more curious about that.  The point rail has a sharper edge than the formed ordinary GarGraves-type rail, so it makes sense from that standpoint.

Center of gravity was found by balancing the loco by itself (no tender) on the 3rd drivers on a craft stick placed crosswise, which caused it to tilt decidedly onto the rear drivers.  Not the least iffy, but also no more scientific than that.  Likewise with new weight, same test placed it decidedly on the 2nd and 3rd driver sets with the same test process and two craft sticks.  No further information to offer.  Others may want to do this in a more precise way to satisfy themselves, no argument, but for me it's good enough and I'll leave it there.  I'm not seeking arguments about the process, just hoping to find others who have had the problem and, if solved, how they went about it in actuality.  It's clearly a rare and quirky issue, devilish to duplicate, but Lou indicates it is not unique to me.  I'm personally done with it with reasonable certainty, but curious about other experience solving it in someone's actual practice.  If someone added sheet lead or other material, enough to solve it, and saved the smoke feature, that would help somebody else for sure.

Update on two things:

I was wrong about flange depth.  The driver flanges on this loco are ~2.2 mm deep, vs my Lionel postwar Berkshire, ~3.2 mm.  Thinking about baseline situation, with rear 2 driver axles carrying weight over a fairly modest “hump” in the track, it wouldn’t be hard for the 2.2 mm flange on the lead drivers to be lifted clear of the railhead so it could come down on the wrong side of the rail after that hump in the track.  Did not directly see that happen, but it may account for the problem.

Pilot truck derailments continued often enough to be a nuisance.  I noticed there seems to be significant side to side slop allowed in the rear (pivot) end of the truck.  That could allow the axle to run at a larger angle to the rails when it is caused to do so for whatever reason, so it might find a flangeway or frog to pick.  I tried making a spacer out of a 1/4" nut drilled to 7/32", filed to fit on each side, plus on the rear-end side so that both screws lined up when done.  Now there is miniscule side movement.  A number of circuits at track speed of 45 mph this morning with no derailments.  That slop may be needed for some tighter curves. My minimum radius is about O62 (31" r), and it doesn't seem needed for me.  Photo of the fix below.  The difference with Lionel Berk doesn't look huge, and most folks obviously aren't having a problem, but this seems to be my solution.


Images (1)
  • pilot truck pivot spacer: pilot truck pivot spacer
Last edited by cnwdon

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