After you're in the hobby awhile and buy trains from various out of state dealers you'll likely get to know the dealers who go the extra-mile and pack the train boxes in EXTRA-cushioned out boxes. They've likely learned, as have I, from experience that the shipping cartons from the importers are not enough to survive real-world handling & shipping. There's three dealers that I know of, and whom get the Lion's share of my business, who most of the time, use outer packing in addition to the shipping carton. Sometimes however, even extra cushioning doesn't help, but it certainly reduces the rate of damage.
From USPS - bullet #1 is key:
Preparing Your Box
Pack your box to protect your contents and to make sure the box arrives intact.
- Use a sturdy box with enough room to add cushioning for fragile items and to prevent items from shifting.
- Tape your box so that it closes flat on all sides and reinforce the flaps with 2-inch wide packing tape.
- If you’re reusing a box, make sure all old logos, shipping labels, and addresses are covered or blacked out.
- Pack your box to protect your contents and make sure the box arrives intact. Tape your box so that it closes flat on all sides without bulging and reinforce the flaps with 2-inch wide packing tape
The cartons themselves, and the amount of cushioning are not the problem.
The problem is that you have multiple-pound (kg for the European folks on the forum) locomotives with very fine, and delicate detail on the outside of them. If *anything* stiffer than the delicate parts comes into contact with it when the locomotive shifts during shipment, it will deflect the plastic/brass/etc.
You can't rely on the shipper to treat the box like a crystal vase all the way to your door. Heck, they come across the ocean on pallets inside of intermodal containers. I'm pretty sure the crane operators and the gods of the sea couldn't care less what's inside.
The key to packaging things of this nature is twofold.
First you need to find sturdy mounting or bracing points and ensure that the packaging contacts the locomotive ONLY in these points. Good candidates for this are the metal fuel tanks, driving wheels, metal frame, metal sand and steam domes, and the thickest part of the plastic shells where there is limited or no detail. Someone else mentioned a screw mount like smaller scales. That's not a bad idea.
Second, there has to be nothing but AIR around the delicate parts. The amount of space required is dependent on the weight of the loco, the stiffness of the packaging material, and how much shock the item can take before the packaging yields to sacrifice it's integrity for the item inside. These are all done in a balance of size, cost, and weight. The typical foam and plastic wrap that accompanies these inside the Styrofoam cradle is actually detrimental to fine detail. Sure it can prevent scratches, but it's not going to save a 030 mil piece of wire or plastic when a 4 lbs engine comes screaming at it.