You're probably thinking of

You're probably thinking of the modern situation in which even the "regular" plastic locos are largely available through some sort of pre-order system. And even most of the ones that you perceive are not like that actually ARE like that, it's just that your dealer is speculatively pre-ordering and then selling them to the public. Almost all of the manufacturers are now on limited-run, generally pre-order production basis.

But to your question, certainly through the early 90s you could reasonably expect to find various bluebox Athearn, Roundhouse, AHM, RTR locos, alongside produced-for-stock Bowser kits, and the like. You could go into most hobby shops that catered to railroading and find some of them, if not many of them.

But even then, the pre-order system was largely in place - but for brass. Someone, whether end consumer, hobby shop, or dealer (and generally some combination of all of these) was placing a firm order with the importer, who was placing an order with a Japanese or Korean workshop to produce a limited run of such-and-such locomotive. I only managed to assemble enough nickels to do that once, in 1990, right before I took the common sabbatical from railroading due to wife, career, kids, etc. I'm sure that I had it before we picked up and moved house in 1991, so a few months elapsed. And I still have that loco, although it is now awaiting a DCC decoder to be able to run on my DCC-only layout.

Fast forward to today. Depending on which manufacturer and which model, you may be waiting for as little as  a few months and possibly for a long time. Some of the Intermountain locos have been in pre-order state for quite a few years. On the other hand, there are a fair number of Intermountain units out there, they obviously didn't take years to produce. Rapido and Atlas seem to have a clearer understanding of their demand and processes and as a result orders you place now probably will arrive late this year. Athearn, Walthers, etc seem to be somewhere in the middle.

90%

of what locos and rolling stock I have bought has been pre-ordered.  I was burned once by a charlatan hobby shop owner who took deposits for the CN PSC Caboose and then went out of business and nobody heard from him again.  It was going to be produced by Rapido exclusively for that shop.  I actually talked to him the week before he vanished and he said everything is fine, no concerns.  True Line Trains ended up buying the assets of the hobby shop, which included the tooling for the caboose and ended up producing it in a very limited run.  Atlas now owns that tooling and everyone is hoping they will produce a run of them.  That's the worst case scenario.

In other cases, I've had pre-orders for things that still have yet to be produced and some that most certainly won't be.  I've waited years for things to finally show up.  I fully trust Rapido and Atlas to produce fairly close to their estimate now, so I'm not worried about putting down a deposit for those companies' items.  I won't for others until I hear they are in production (which may be too late, as I've found out).  

The reality is, if you don't want to pre-order, you may be out of luck for that loco you really want, or end up paying ridiculous prices (those cabooses mentioned above I've seen go for over $300).  All in all, the pre-orders don't bother me.  If manufacturers don't have that certainty of orders, they can really be hurt and we want all our manufacturers to be strong and healthy.  Its better for all of us.

At least since the 1970’s ...

I recall regularly seeing ads in MR and RMC for “see you dealer to reserve” for Brass Tenshodo, Pacific Fast Mail, etc. All the time when I was a kid.

Rapido works this way entirely,  it they only charge a buck to pre-order - you don’t have to pay up front, and they are really good about giving a couple of weeks notice of impending arrivals and credit card charges.  They likely apply a small percentage on top of pre-orders for direct sales or last minute orders (and warranty), but I would bet anything you see on a dealer’s shelf was pre-orderered by the dealer based on their own knowledge of the local market, plus requests from customers.
 

A bunch of FP9’s showed up at Central Hobbies a couple of weeks ago and they weren’t selling them until they got their pre-orders for customers set aside.  I had already ordered mine from Rapido because they said they were having trouble getting stock.  

No one outside of maybe Athearn and Rivarossi can afford the cost of doing a spec model. Most are runs of locomotives already produced at some point so I would guess it’s a crap shoot on how many one can guess they would sell, and if the ore-order volume isn’t there I think they wait until it is to justify the cost of mold making.  They don’t seem to make much extra though as they sell out quick.

You have to decide if you want to support a dealer or go direct.  With Rapido, I per-order now to make sure I get exactly what I want, and then if the dealer has some extras and I want more of something, those are impulse buys.

I think it’s safe if you stick to reputable dealers and manufacturers,  it personally, I would be cautious these days of having to provide too much of a down payment.

I used to work in a Hobby

I used to work in a Hobby shop in the late 80s and early 90s and what folks didn’t know is that everything was limited production even then.  Just not as limited as today.   Back then they made more and sold them to hobby shops in ones and two to put on the shelf and distributors such as Walthers bought tons to keep on the shelf.  So the customer did realize that years could go buy before the given model was reissued in a give paint scheme.

The big difference is an engine selling for 40,  cost the hobby shop 30 and the distributor 20 so you can put 10 of them on the shelf or a hundred in your warehouse.  Keep in mind that I am sure the modern manufacturers would love to produce enough to keep them on the shelf at the destributers for 5 years,  it is just the distributor won’t sink that much capital into a give locomotive.

We expect turnover on the hobby shelf and in the distribution warehouse to be much more rapid then we did in the 70/80s.  No one would put up with those shelf’s full of 10 and 20 year old parts and models anymore.

-Doug M

... what folks didn’t know is

I always thought that was generally known to avid model railroaders for a long time, as in since the 1980's anyway.

Pre-ordering context

Thanks for those comments that help in understanding the context for locomotive/rolling stock supply and demand.

Reading the comments it makes sense that pre-ordering has always been a part of the hobby at some level, even if by hobby shop owners and distributors before it was individual modelers. Both manufacturers and sellers of niche items would want some assurance those items were wanted before sinking capital into what, on a macro scale, is really a low demand product.

It reminded me of a comment I heard several weeks back from a long time hobby shop owner. He was saying that many years ago if he was reviewing the books and had some working capital that he would simply phone up Athearn and ask a rep to send $400 worth of freight cars and maybe $100 worth of passenger cars. He left it up to them as to what showed up. He just asked for an assortment.

He said that these days he has to be more circumspect about his ordering, and very specific about what is wanted. 

Changing Times

The hobby has changed. It is now likely that I can order almost any prototypical diesel in any paint scheme, with sound and DCC. The details are prototypical and all hand-applied. This all costs money. Instead of the older days, where you'd push out a GP38-2 and you might allow for a non-dynamic brake option, you now have to worry about rivet counters who will reject your model if the hood doors are 3" too short, or if the bell is in an unprototypical spot for that particular prototype.

Mass-producing used to work well, until modelers started demanding more from their models. Or, the manufacturers realized that they could increase the price to over 100% for the same model from 10 years ago. It's kind of a chicken-and-egg situation.

Also, with the way advancements are made, you don't want to be on the hook for having the last release when a new one comes around. 6 years ago, the big thing was DCC, sound, and LED headlights and ditch lights. Now, the bar is increased to where you have DCC, sound, keep-alives, and lit numberboards, walkways, and steps. Who is going to purchase an Athearn Genesis NS ES44AC from 5 years ago for nearly the same price as a new one with the whole lighting package?

Who is going to purchase an

They're not going to be the same price.  ES44s from 5 years ago listed about $60 less than the latest run of C44s.   I find myself tracking down a lot of older stock when I can, as list from 5 years ago is cheaper than discount today (I made out buying old stock Genesis PC&F boxes and FGE reefers vs the exact same things released over the last two months).  In the case of the locomotives, I'm see some of the features you mentioned hitting a point of diminishing returns.

This is all connected

This goes back to my argument elsewhere that the hobby is moving towards specialization.  We used to buy whatever engine was made in whatever paint scheme was close to what we wanted.  And we bought any car that sort of fit the time frame we modeled.  So it was relatively easy for a hobby shop to keep stock and make sales.

Today we not only want a hopper car (for example) but the exact one with a given specific set of details.  I tried to get A company to release a C&O caboose in the paint scheme used on the first generation of steel caboose as they have a model of the second generation of them.  They said they wouldn’t because the roof walks on there model were steel vs 1st generations Wood, and the hand break on the model was a vertical mechanical system. Vs an older style horizontal.  Back in the 80s or before there is NO WAY a manufacturer is going to pass up painting a model they have in a scheme that it is 90% correct for.  Does anyone remember all the paint scheme for different railroads that Athern painted the Santa Fe caboose in?  It seamed for a while that was the only caboose available and was sold painted for every railroad on the continent.

You couldn’t get away with that today.

This huge variety of models and everyone (well most anyway) wanting specific  models is in my opinion one of the factors that ran the local hobby shop out of business.  It just became to hard to stock enough verity of models to please the local customers.  And it was in general to hard to reach far enough to get enough customers to support this increase in cost of doing business.

In the world in general much like history things do not exist in a vacuum.  One thing relates to the next and the next and so in and so forth.

You can argue chicken vs egg but the reality is technology has advanced so that we A) have access to information so we know enough to understand what a given prototype piece of equipment should look like (thanks the internet) but this started before that with specialty magazines and Historical societies, andb) the ability  to make these very accurate models in small enough batches to make them profitable with the limited customers that a spicific model would attract.  We see this starting with laser cut buildings.  But eventually cars and engines joined the parade.

One of the first modern well preforming steam engines was the Bachmann 2-8-0.  It was a very generic 2-8-0 and it ran great and sold like hot cakes.  But most engines after that are prototypes not freelanced (with a few noticeable exceptions). 
 

So this is all related.  
 

-Doug M

On the way to the Star Trek replicator ...

... we will soon be able to purchase the right to use designs and print our own locomotives and rolling stock!

... we will soon be able to

Have you heard of " Thingiverse"?