I've changed my mind about the AD60. After doing a fair amount of research into Garratts, I've decided I'm going to start at the other end of the Garratt timeframe.
The first one ever was the 1909 Tasmanian K1, but that wasn't for main line operation. The first Garratts for main line operation were the West Australian Class M, which were built in 1911 but didn't enter service until 1912.
The Class M were 2-6-0+0-6-2. From what I can gather, it seems that Garratts that lacked leading and trailing wheels each side of the drivers were not as successful. Apparently the lack of guidance caused track wear problems, particularly on corners, and could even result in derailing if it got too bad. Also, since one drive unit was always running "in reverse" it tended to lack stability if it had no leading wheels. Because of this, the vast majority of Garratts were built as 2-6-2+2-6-2 or 4-8-2+2-8-4 or whatever. So, those are the types I intend to model. Might as well do the useful ones.
The first ones that had wheels in all the right places where the Tasmanian Government Railway Class L (2-6-2+2-6-2) and Class M (4-4-2+4-4-2). These are the ones I'm going to try first. They share exactly the same central unit, so in terms of modelling work I'll get two for the price of one. The central unit has shapes that will help ease of modelling too. They were also kinda cute and nicely proportioned, which is always a bonus. Even better, I've managed to track down the original 1911 plans from Beyer Peacock.
These are good ones to have as functional units. The Class L 2-6-2+2-6-2 was a freight locomotive, and was basically the same unit that Beyer Peacock kept producing for smallish freight/mixed Garratts right through until the end of steam. Most of the later 2-6-2+2-6-2's were just minor variations on the Class L, for the simple reason that they pretty much got it right the first time. They were good little grunters, easy to look after, good on fuel, and very durable. So, if I make one of those it'll be useful right through the rest of the steam era.
I'll have to re-gauge it from Cape to standard to suit the scale/track in RRT3, but that applies to most Garratts anyway so I might as well get into it. I've taken a preliminary look at it and don't think it will present much of a problem. It can't just be scaled up by a factor of 56.5/42, because that would make it stupid large. Seems like the way to do it is some subtle cheating on the widths of front and rear units, with possibly slightly wider footplates on the central unit. That will allow moving the cylinders out. I may also slightly cheat cylinder bore size if it needs it.
The Class M is an interesting one. That was an express unit, and was the only 8 cylinder locomotive ever built. Four cylinders were outside in the usual places, and the other four were inside in inconvenient places. The inner ones used a rocker arm to actuate the valve gear. This all made it a nightmare to maintain, but as far as I can tell it wasn't unreliable as long as it was maintained. The reports I can find just grumble about the difficulty and cost of servicing, but don't grumble about it breaking down. It seems to have been reliable, with good speed and good haulage ability.
It actually set a world speed record for Garratts in 1912. Basically they just bolted it together, rolled it out onto the tracks, and promptly clocked 55 mph on a test run.
The Railway Gazette, 7 December, 1912 wrote:It is understood that as a result of a test run made with the Garratt passenger locomotive, referred to above, a speed of 55 miles per hour has been attained. The trial took place under the supervision of the Chief Mechanical Engineer, Mr. W. R. Deeble, on the section from Launceston to Deloraine, Tasmania, and was highly satisfactory from every point of view. The engine steamed and curved with exceptional steadiness and the speeds were registered by an automatic recorder.
Ok, so we gotta have one of those. Who wouldn't want a 1912 sports model Garratt in the pack?
Since a lot of the modelling will be the same for this and for the Class L, I figure start the Garratts off with one good freighter and one good express. Provisional plan is availability from 1912 onwards, with maybe a 1939/40 end date for the Class M and the L class carrying through until the late '50's.
I think the Class M's reliability should be modelled as Above Average, or possibly Good, but with much higher maintenance costs than the Class L. Class L reliability should be at least Very Good. Fuel economy for both should be decent. At a first guess, Average for the Class M and Good for the Class L sounds about right. Free weight and pulling power adjusted to suit, of course. Passenger rating should be Acceptable for the Class L, and just has to be Ultra Cool for the Class M.
I think I'll make top speed for the Class M 60 mph, on the basis that if it did 55 straight off the showroom floor it was probably was capable of a bit more on a good day. There are unverified reports of it topping 60, and it seems quite plausible. Top speed for the Class L should be around 40. That makes sense, based on equal steam capacity for both classses, differences in driver diameter, etc, etc. Acceleration of Above Average or Good for both sounds about right, given that they are basically double heading* a Prairie or an Atlantic, so they aren't going to be slugs when lightly loaded.
Anyway that's the plan.
*By the way, since Garratts don't have separate tenders I cannot see any reason why it wouldn't be possible to do double headed Garratts, just like diesels and electrics are done. Not sure anyone will want to, but it should be possible as long as the Garratts can be modelled via one body and two trucks each (ie: not using up the tender slot).
Any kind of user made Beyer-Garrat would be most welcome in this game. But I think it really demands graphic skills to "build" it. Some kind of Beyer-Garrat would be useful in scenarios from Australia, South America, Spain and especially Africa. I really hope someday someone can create one 
