Dear ???

The quick answer would be "whatever knuckle couplers the locos and cars came with, saves $$$ and conversion time".

However, a look to the guys who have trod the "big train" path before you
- La Mesa Model RR club
- "Arid Australia" world-record-holding HO iron-ore train show layout
- et al

would point to the need for:
- metal couplers
- body mounted couplers
- _properly_mounted_ couplers
- and _Consistent_ height/performance/coupler swing
(not to mention other key factors such as _consistent_ car-weighting and wheelsets)

which, when distilled-down,
seems to perfectly match the product description for the long-serving Kadee #5 coupler...

http://www.kadee.com/htmbord/page5.htm

NB that for such "fleet missions", all Kadee couplers are avaiable in "bulk packs",
saves $$$ and in some cases avoids buying un-needed draftgear booxes...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

​PS couplers are not the only factor that needs to be mastered to get reliable operation of long "unit trains".
Locos, cars, couplers, weight, the layout, trackwork, geometry, speed, and train-handling all play a part...

PPS for some "unit train" missions where storage and transportation issues permit,
using rigid drawbars (formed brass tube?) between pairs of cars can be a worthy exercise.
It halves the number of "real couplers" required for a given train, works just as well in terms of "running the train", and doesn't unduly affect operations (Unit trains are rarely broken-up and car-by-car switched...)

Kadee period. Buy in bulk packs with out draft gear boxes, saves money. Easiest way to find one of the other brands is to have it behind the motive power with a heavy train going upgrade. They fail, kadees do not in my experience, all power on head end 180 heavy cars in tow no issues on my way to 200 cars. In 1959 the Virginian and N&W pulled trains like this with three units.

 Use Kadee #148, not #5....  The 148 don't use the little copper spring, but a built in whisker.  Saves lots of time on the conversion and also available in bulk packs.

for that much weight you need the all metal versions.  The height needs to be right.  A multi pack is the way to go for sure.  

#5 multi packs come with extra springs.  #148 multi packs do not come with extra whiskers.  

Kadee is great, but not perfect. Periodically, one or two per 100 are either missing a whisker or they come off while installing.    The shank on the whisker version is thicker at the attachment.  Because of this, they tend to require more washers to set the height.  

For the last two reasons, I'm with the Prof here, I would choose the #5.  A broken or missing whisker requires a return.  The flat springs tend to accumulate like weeds, no need to return anything.

Dear MRHers,

Have to say I've never had any issues with the phosphor bronze centring springs on #5s in over 3 decades,
so haven't personally felt the need to jettison them for the "whisker" versions.

Main reason I suggested #5s (apart from the price, $139.75rrp for 100pr #5s VS $172.60rrp for 100pr #148s)
is that the OP didn't mention anything about preferring scale-sized coupler heads,
and more-importantly didn't qualify anything about curve radii or track-installation conditions the "unit-train" would have to overcome.

Given this lack of info, and the inferred need for the unit-train to stay coupled up-hill-and-down-dale,
thru whatever may be thrown at it,

the slightly over-sized coupler head/knuckle of a #5 provides more gathering area and cross-sectional strength,
and a lot more tolerance to up-down/high-low/over-underride conditions,
esp when under extreme pulling (tension) or buffing (compression) forces...

Having hauled actual-kilograms-worth of both HO and On30 trailing loads with #5s,
I've yet to have a _properly_installed_ #5 snap a knuckle or over/under-ride in show-operations conditions...
(whereas other "plastic knuckles" have rapidly disintegrated).

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS James, happy to have someone of your calibre respectfully disagree with thoughtful reasoning,
I'm regularly wrong about many things, and made my peace with that fact many moons ago... :-)

Kadee will discontinue the number fives and replace with only whiskers per their web site. I have had to replace the couplers with the separate bronze spring in some applications due to slop. In the cases in which I did the thicker whisker coupler shank filled the box better and exhibited much less play. I am almost positive that since I have typed that someone will have an issue that is better served by the old number 5s and it will likely show up next. I use both but only genuine kadee brand.

This is the message I received from them:

"We'd certainly like to know who starts these rumors. We have no intentions
of discontinuing the #5 coupler or the phosphor bronze centering spring.
Although the tooling for the spring is wearing out, the introduction of our
whisker couplers has reduced the amount of springs we have to produce which
adds some longevity to the tooling.

Sam Clarke
Kadee Quality Products"

I am relieved because I do prefer the older versions.
 

A friend of mine loves to run long 100+ car trains. He also has a long 2% grade on his layout. One day, testing a new locomotive he started the train. The last engine of the consist was the new engine and it came with plastic couplers. Everything else had metal Kadees. As the locomotive consist reached the top of the grade, the plastic coupler's shank flexed and down came 100+ cars all with metal wheels and Kadees. The grade was straight for about 30 feet with a 32" radius curve at the bottom. You can imagine the devastation. The roar was impressive, the damage even more. This one plastic coupler cost him real dollars in repairs and replacement of cars. No matter the cost, metal couplers should be used exclusively for a variety of reasons. Even on my small switching layout I convert everything to Kadee because I find they couple and un couple reliably with my bamboo skewer.

As always, your mileage may vary but consider this story when considering cost.

Or should I say my CAT broke it <grin>.  He knocked a box of engines from a bench they were on.  The only damage was a single coupler that snapped across the shank and it was a genuine #33 Kadee.  Fortunately, I had to buy 4 pairs to do the 3 engines, so I still had an extra pair (or should I say now, an extra coupler).  (Is it any wonder we call that cat Random Havoc?)

>>PS James, happy to have someone of your calibre respectfully disagree with thoughtful reasoning,
>>I'm regularly wrong about many things, and made my peace with that fact many moons ago... :-)

>>Prof Klyzlr

Their is no reason for a lack of respect here.  Both of us are expressing an opinion.  My opinion is based upon as much of watching scouts try to use different couplers and how much trouble they had vs all other aspects.  In the first MBA sessions I had a bunch of spare McHenries that I had removed from my cars.  In the later sessions, Kadee supplied #148 couplers.  I am certainly not going to go back thru all my previous cars and swap out #5 couplers for #148 couplers, but in the same measure, I don't buy #5s any more.

And in the long run - which is worth more - my time or a few dollars?  And who is to say which of us is correct?

Oh, here is an interestign tidbit for the crowd.  One of the local clubs (not mine, but another) is swapping out #5s for #148s wholesale.

As the locomotive consist reached the top of the grade, the plastic coupler's shank flexed and down came 100+ cars all with metal wheels and Kadees. The grade was straight for about 30 feet with a 32" radius curve at the bottom. You can imagine the devastation. The roar was impressive, the damage even more. This one plastic coupler cost him real dollars in repairs and replacement of cars.

We were having a discussion the other day about this type of event and solutions were everything from air jets to slow the cars to a run-away track that switches the cut into a big tub filled with soft insulation.  My solution was a speed trap that when it senses the speed and length of the cut, it activates an auto dialer calls the hobby shop and places a order for new cars.