USRA Light Mountain 4-8-2 (2018v2) vs USRA Light 2-8-2 (2020v2)

This sounds vaguely familiar; didn't someone recently ask about this? I seem recall it was pointed out that a Big Boy is barely 30 inches in length, so it was decided that the "30 inch Mikado" was a typo...I think.

Mark in Oregon

The min radius isn't a function of totally locomotive length.  We don't know if that locomotive length includes the tender or not.  And it's also not even a function of the length of just the engine, it has to do with the size of the drivers, and the size of the tender trucks.  With locomotives that have the same number of drivers, the one with bigger drivers will require a greater turning radius as the front and rear axle are farther apart.  Also how the leading and trailing trucks are hung will affect the radius the train can use.  The Big Boy was mentioned, the scale locomotive it self will handle O54 track.  On the other hand the tender will not so the locomotive as a whole has a bigger radius requirement.  In the end this is all geometry, that maths stuff you learned in HS but thought you'd never need.

I have a Lionel Mikado and run it on O36 with no issues.  I do not have a Mountain so I can't speak to that one.  But from looking at the two pages you linked, the drivers of the Mountain look bigger so I would expect it to require a larger radius turn.

S Runner,

    The light mikados in the 2017 catalog have a length of 20 1/4”. The 30” is a typo probably should be 20” if they shortened the tender gap a 1/4”.

JohnB

@JohnB posted:

S Runner,

    The light mikados in the 2017 catalog have a length of 20 1/4”. The 30” is a typo probably should be 20” if they shortened the tender gap a 1/4”.

JohnB

It's the engine/tender gap that's going to cause issues on tight curves with either a light mountain or the light mikado, more than driver size or how much the pilot and trailing truck swing.

The driver size does increase the rigid wheelbase, which is definitely a limiting factor.  If you're determined to run the new Mikado on smaller curves, you might be able to increase the engine-tender gap.  For example you could replace the drawbar with the longer one from the earlier version.  But you can't do anything to shorten the Mountain's wheelbase. 

Three-rail locos have typically used one-piece side rods, and this also limits how "flexible" a large steamer can be.  I'm not sure why Lionel and MTH don't use individually segmented rods.  But especially before speed control, a lot of my 8-drivered locos would slow noticeably in sharp turns.  You could see that there was almost no slack in the outside driving rod, everything was maxed out.  If you want big locos to run comfortably, the only sure-fire solution is wider curves.  

Strummer - Yes, this issue came up in the recent Post 'Lionel 2020 Volume 2 catalog is up. Whatcha lookin at?' And the consensus is - as JohnB indicated above - that it's a typo.

Appreciate the replies/feedback. 

In addition to your replies, I subsequently found previous Lionel production of Light Mikados in the 2017v1 catalog where they are spec'd at length of 20.25 inches and minimum curve of O54.  I expect the 2-8-2 Light Mikados in the 2020 catalog will be the same. 

Tom

Weaver had done a three rail brass Pennsy mikado early 1990's  Most of my layout is O54.  

S-Runner - Even further back than your 2017 Mikado reference the 2012 Signature Edition catalog shows the length of several Mikados displayed to be 20.75 inches and a minimum curve of 0-36.  However, I have one of those engines (6-11258) and I can confirm it will run on 031 tubular track curves, although it's not very pretty from an aesthetic standpoint.

Something else I've either read or discovered myself over the years is that occasionally Lionel - when advertising an engine with matching cars - will state a larger minimum curve than required for the engine itself due to the matching over-sized passenger cars.  Sometimes, however, they don't state the minimum curve for the engine and cars individually.  

In conclusion, for those of us who have small layouts with tighter curves determining an engine's minimum curve is critical.  Otherwise you run the risk of purchasing an expensive engine that you fully intend to run, only to find out that you can't run it on your layout because it derails!