It is funny how people with any technology or change have an all or nothing attitude with regards to technology and tools and such. Injection molding works great for some applications, not for others; with metal work, milling works great in some cases, forging or casting in others. Things that once had to be made almost by hand can be done by CNC machines at a fraction of the cost.
3D printing is a relatively new technology, and yet it has improved tremendously, and it has its role. For hobbyists, machines in the range someone at home can order could potentially allow that person to make detail parts, lamp posts, potentially window frames and the like, things of that nature, and depending on how they actually print could come up with some pretty fine scale things, if they are willing to do it slowly, it is just another tool in the shop, I can print a window frame on a 3D printer or make it out of plastruct or something of the same, or make it out of wood using an exacto knife and strip wood.
Is it likely that a manufacturer will use 3D printing anytime soon to mass produce body shells and the like? Even though 3D printing can work with a variety of materials, including metal, it unlikely they would be producing in mass the parts needed for an engine, and even with something like a freight or passenger car there are limits, speed and cost being big concerns. However, they can use it to produce prototypes to check out how it looks.
However, seeing how fast technology evolves there is no reason to suppose that eventually there will be rapid, fine scale 3D printing, and it could take over from injection moulding and the like, they already can print metals, so you could eventually see 3D printing used to make the parts and robotic technology to build the train, there is absolutely no barrier to this happening, and I would argue it would be a suckers bet to say this couldn't/won't happen in the next 10 years, given how fast 3D printing and robots and AI are coming along.
We could get to the point where build to order would literally be that, where 3D printing and robotics could allow you to get an engine or something for an obscure railroad that no manufacturer would produce and I would speculate at a price not all that much different than what let's say the Lionel Vision Big Boy costs.
For today, 3D is a hobbyists tinkering thing and on the high end for use in limited applications, like industrial prototyping, for the most part. 45 years ago the first home PC had 400 bytes of memory, and even the largest mainframe computers had a fraction of the computing horsepower your smart phone has, within 10 years the PC was threatening the power of mid range systems (ask someone why IBM restricted the speeds on the Intel processors their PC AT used, it was because it was threatening their system 36), 10 years after that and mainframes were becoming boat anchors *shrug*. 3D printing today is where PC's were in the late 70's roughly, but the rate of change is not the same, it is accelerating, so who knows? I'll also add that right now it can be difficult to do the 3D programming needed to run the printers, but again look at the computer world. Once upon a time it was difficult to program, but over time languages evolved, and today someone with very little training can design and build complex websites, and there are tools that allow people to write phone apps with very little effort. Eventually, tools are gonna happen where you don't have to use autocad to do 3D design and printing, potentially you will be able to 'tell' the software what you want, and it will do the detail work for you then print it *shrug*
As far as a printer being able to print 'complex scenes' like the one a poster put out there, I don't think so, to recreate that takes a kind of artistry. a 3D printer might help you make the parts,might even print them out in color, but the actual design of that scene and putting it together is more akin to art, and computers and such don't do a very good job with that, they are wonderful tools and aids, but the vision is the artists
