A personal view by Bruce (H&DLR).
The question is complicated slightly by the fact that most garden railways are not an exact model of a precise prototype, so it's not possible to say "this formation precisely replicates the XX.XX departure from Anytown to Somewhere Else Junction". Nevertheless, if you look around this site, you'll see many examples of trains running on imaginary branch lines that still look right. Now, there are a lot of reasons why this may be, but I'd like to suggest that part of that indefinable atmosphere includes running a prototypical or plausible formation.
There are, I think, a number of ways we can approach this.
Often cited as a reason for running anything together. If you enjoy this, that's fine. Some things, however, rarely look convincing. Yes, I'm aware that there are/were sections of dual or multi gauge track where narrow gauge engines shunted or hauled standard gauge rolling stock and vice versa. But to my eye, improbable combinations of 1:29 scaled locomotives hauling 1:20.3 scaled rolling stock aren't visually satisfying - or necessary. There are a number of fine layouts out there where the desire to run all sorts of rolling stock, modelled to a wide variety of scales, on 45mm gauge track work very well on the eye . How? Simple - by keeping matching consists or themes together and avoiding things that jar. See, for example, Marc and Barbara Horowitz' Ogden Botanical Railway, which was designed and built to allow all sorts of trains to run.
For my starting point, I'm going to assume that we're running a train that is modelled to more or less the same scale (whether 1:19, 1:22.5, 1:20.3 or another "garden" scale) and gauge (which may, on the prototype, be metre gauge, 760mm, 750mm, 2', 2'6" - you get the idea). Search far enough, and you'll certainly find some bizarre formations. But there's usually a very good operating reason why, on that railway at that time, that train ran. Most trains, most of the time, fulfilled a humdrum task, day in, day out. I'd like to suggest, therefore, that while the occasional "special" is fine, perhaps the everyday service would look more plausible if it mirrored the more typical prototype operations. Which leads me on to my next point.
Looking at pictures of lines that inspire us can tell us a great deal about operations. I do find it frustrating that so many British railway photographers focus on the locomotive to the exclusion of the train! Fortunately, many German and Austrian photographers take a wider view of such matters.
Very simply, searching (maybe on line) for photographs of favourite railways can reveal a great deal about typical formations. For example, the Austrian class 2091 diesel is a relatively under powered locomotive. So it tends only to haul short trains. But how short? Well, a search around suggests that it's limited to two bogie or four two axle coaches. Rarely do you see a single cl. 2091 hauling any more than that. Rarely do you see a picture of a cl. 2091 on freight. A consist of four bogie coaches may be the norm on the Pinzgau line - but it won't have a 2091 at the head! Photographs can obviously be supplemented by reading books. The text may often enlighten us as to why that consist ran.
So - we've found evidence of the sorts of trains that operate on the prototype versions of our railway. But it might help to understand how these formations are put together and why.
Doing a bit (or even a lot) of research.
Why are these things important? Because they have a fundamental effect on how trains are marshalled and operated.
This might seem arcane, but understanding a little about the way in which train braking has developed helps to explain why some mainland European train formations look so different to British train formations. In the early days of railways, although individual vehicles might have hand operated brakes, once the train was on the move it was stopped by the locomotive brake and the guard's brake operated from his compartment or van. This has a number of fairly obvious limitations, and so continuous brakes which could be applied by the driver on all vehicles simultaneously were developed. While this was useful in enabling the driver to control a heavier train, it was of little use if the train became divided, whether by accident or design. It was not long before the Board of Trade and the Railway Inspectorate in Great Britain were demanding the installation of automatic continuous brakes, at least on passenger trains. With the vacuum brake system, a vacuum generated by the locomotive kept the brakes off; when the driver (or guard) wanted to apply the brakes, or if the train split, air would be admitted to the system causing the brakes to be applied automatically. Such systems were initially opposed by many railway companies on cost grounds, but the clamour became irresistible after the Armagh accident in 1889 when 78 people, including many Sunday school children, lost their lives.
In the British Isles, and on many systems based on UK practice, the automatic vacuum brake became the norm for passenger rolling stock; it was cheaper to install and operate (although less efficient) than air brakes, which became the norm on most other railway systems in Europe and the US. However, the regulations did not apply to freight stock, and unfitted freight stock (goods wagons without automatic brakes) remained in service in the UK into the 1980's, long after the idea of unfitted wagons had disappeared from mainland Europe and the US. Now, whether or not your rolling stock has automatic vacuum/air brakes has a major impact on train formations. In the UK, passenger rolling stock must have continuous automatic brakes operational. It cannot, therefore, be marshalled behind unfitted freight rolling stock, since it is the locomotive that operates the brakes. It must, of course, be marshalled in front of any unfitted freight stock - and it is for this reason that almost all mixed trains that you ever see in Great Britain or Ireland have the goods vehicles behind the passenger coaches. Yes, if the freight stock has continuous automatic brakes, or at least pipes under it for the brakes, it can be marshalled in front of the coaches. Ah, I hear you cry, what about the prototype for everything? Well, there are some... the Cavan and Leitrim Railway among others in Ireland, or the Leek and Manifold and the Lynton and Barnstaple in England.
What about railways whose development was not influenced by British practice? In mainland Europe and the US, mainline railways generally moved to air brakes rather than vacuum brakes. This necessitates a Westinghouse (or similar) compressor on the locomotive to create the necessary pressure, whereas the vacuum required for the vacuum brake can be created by the ejector on a steam engine. However, while the air brake is now the de facto standard, German (and some Austrian) narrow gauge lines used the Heberlein brake for many years. This too is a continuous automatic brake... but has a continuous cable along the train rather than an air or vacuum pipe. This was superseded by vacuum or air brake in time. But probably the key difference in most cases is that all rolling stock - freight and passenger is "fitted" or braked. This means that the order of the rolling stock is, subject to one or two other operational considerations that we'll come on to, all but immaterial. On the Rhätische Bahn in Switzerland, you see what are, to British eyes, some very strange formations...
Mostly, coaches are heated from the engine, historically by steam from a steam engine. Even in early diesel days, diesel locomotives for passenger trains had a steam heating boiler. Later, this was done by electric train heating (now, in the days of air conditioning, called electric train services, but it's the same idea). So coupling your locomotive to your passenger coaches irrespective of whether your freight stock is braked is still a good idea. Again, there are vehicles - parcels vans in particular - where steam heat or electric services can be channelled through to the passenger coaches. Equally, on some lines (for example in Saxony), heating in the carriages was provided by a stove or two, guard's van style, obviating the need for the coach to be next to the engine.
So... the train above has continuous automatic brakes which operate on the open wagon and the coaches, ensuring safety. The coaches each have two stoves in them, so there's no need for steam heating from the locomotive. And there's a brake van/parcels coach at the end, so the guard can tend to the stoves and look after the passengers. There's a winter photograph in the Verlag Kenning monograph on the Pöhlwassertalbahn showing a formation of locomotive/open wagon/passenger coach/open wagon/passenger coach/brake van. This is workable, if you follow the logic outlined above, but the author notes that it is unusual and that the passengers would be glad of the stoves in the coaches!
You will find photographs of the German standard gauge V60 diesel hauling the 6 wheel passenger coaches, all modelled by Piko. Just be aware that such movements are exceptional summer only operations when a more suitable locomotive equipped with train heating (V100 or V160) could not be provided. Or it's shunting empties.
Some things are permitted, others aren't. This does vary from railway to railway, but on British based systems, shunting passenger trains with the passengers on board is usually prohibited. The coaches would be "parked" (securely braked, obviously!) in the platform if the engine needed to trundle off and shunt some freight. In addition to General Instructions, there would also be local instructions, specifying how operations were to be conducted at a certain location. It is because of this that there's usually a "prototype for everything", and a little searching may well find a modus operandi that perfectly fits a similar situation on your railway.
Often these instructions follow on from incidents, not necessarily on the line in question. However, at Oberglobenstein on the Pöhlwassertalbahn in Saxony, passenger coaches were specifically not to be left on the main line, which happened to be on a gradient. You can guess what happened in the early days...
On freight trains, hazardous goods used not to be marshalled next to the locomotive, especially a steam locomotive. A barrier wagon would be provided - which could simply be a vehicle not carrying hazardous goods. By the way, an empty (discharged) petrol tanker is more dangerous than a full one, so pretending that the tanker is empty won't work. It's the fumes.
The one that provides most of the unusual train formations. Providing it's not prohibited, trains may be formed up in such a way as to facilitate shunting etc along the route. This can be especially relevant where sidings face or lead off in a particular way. Fairly obviously, freight trains would be marshalled (subject to any braking requirements) to allow segments to be dropped off at sidings and yards along the way.
Sadly, all too often this featured last on the list. However, it exerts a considerable influence over the formation of passenger trains.
Brake compartment/van/coach. This would tend to be marshalled at one end, so that (in one direction at any rate!) the guard would have a good view along the train. On a corridor or gangwayed train, it also means that passengers do not have to walk through the parcels and mail areas, enabling them to be kept firmly locked and out of bounds while still allowing passengers to move along the rest of the train. Blocking access in the middle of the train is not a good idea, since it may mean (APT-style) providing two of everything - first class, buffets, catering staff.
Buffet cars would often be provided towards the centre of a train - generally used to divide first class from second class.
On main lines in Britain, first class is normally at the London end of the train, offering the shortest walk to/from the exit for the largest number of fist class passengers. Obviously this is at the whim of operating convenience, since if the train reverses en route first class gets reversed too.
Trains that divide en route make life much more complex, especially if both first and second class are to be provided in each portion. Once common on long distance international trains, where coaches from one train (with passengers on board) would be detached and shunted over to be joined on to another train.
Special excursion trains. Not as rare as you might assume, given my comments above. At the time of the Beeching Report, something like 40% of British Railways' coaches made only a few trips per year - in other words, there was plenty of rolling stock lying around for that excursion.
Most trains on the Austrian Steyrtalbahn consisted of no more than two bogie or four x four wheeled coaches plus a van. So what on earth are all these coaches doing in Garsten depot? Waiting for a special. At the little halt of Unterhimmel-Christkindl there is a pilgrimage church, to which the Steyrtalbahn ran frequent specials to take the pilgrims from the interchange with the standard gauge.
On the next page you'll see some ideas...