This is based on the current discussion in another thread re: sending Polish planes to Ukraine. Flying them in seems a non-starter, as does moving them on a road. Could someone in theory conceal a jet plane on a train and transport it that way?
Some assembly required.
^ I’m picturing this with the Space Shuttle perched on top and someone exclaiming, Hey, that’s not right!
Why not just send Ukrainian pilots over to Poland and pick them up? Fully loaded of course.
737s
Imagine how fast that train could go if the engines were still attached.
If you insist.
Because,
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The planes would still be vulnerable to Russian interdiction on their way back into Ukrainian airspace,
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That would make Poland a pretty direct party to the conflict, and Russia might well attempt to interdict the planes on the ground, in Poland, and might widen that to a wider campaign against Polish airfields to stop them from providing more airplanes,
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It would give Russia a pretext for attacking Ukrainian refugees fleeing into Poland, since some of the Ukrainians crossing into Poland would demonstrably be legitimate military targets.
And, this illustrates part of the challenge. Train cars aren’t particularly wide – in most of the world (including Europe), the distance between the rails of a train track usually follows the “standard gauge” width, which is 4’ 8.5".
The bodies of locomotives and train cars are wider than that gauge (which determines the distance between the wheels), but not by a lot – in the U.S., most cars aren’t much wider than about 10 feet. Many train tracks may pass through areas where the horizontal clearance to either side of the tracks may not be a whole lot wider than the width of a train car.
The MiG-29 (the model of plane which is in question in the case of Poland and Ukraine) has a wingspan of 37 feet, 3 inches (source), which means that it would be severely hanging over the sides of a flat car it was placed on (by close to 14 feet on each side), and quite possibly running into obstacles along the sides of the track as it was transported.
Plane to Busan? I’unno!
But going through the tunnels would be…impressive!
Just cut a notch for the wings in the walls of the tunnel! Duh! Do I have to think of everything? 
Allow me to present to you the experimental M-497 “Black Beetle” railcar, developed by the New York Central Railroad in the 1960s. They attached two jet engines from a B-36 bomber to the top of a Rail Diesel Car. It still holds the U.S. rail speed record, of 183.68 mph.
Probably not very fast. In the US, a fully loaded train can weigh somewhere around 24 milllion pounds. A Boeing 737 MAX makes about 60,000 pounds of thrust. Best forward acceleration for the whole assembly, 0.0025 g. For comparison, a decent locomotive can deliver 150,000-200,000 pounds of tractive force, and for a 24M-pound train, it’ll be running with four or five such locomotives.
Actually, Ukraine and Russia have 5’ broad gauge rails.
They run to the Polish border. Not much won but every inch count, right?
Could you mount them on a train if you rolled the MIG 90 degrees sideways?
Or build a tunnel under the border?
Gliders have removable wings that can be placed parallel to the fuselage in a long, narrow trailer; unfortunately, with very few exceptions a glider is not a MiG-29. US Navy planes have wings/ wing tips that fold up &/or back to make for more compact storage on carriers, so yes, it could be done but you’d probably need a fighter designed to get that compact & not just do it with an existing plane.
Red Bull give you wings!
Could a ship be trained by a plane?
During my time at Boeing, I have seen thousands of those fuselages brought in by train. At the height of our productivity, just prior to the assembly shut down because of the Max plane crashes, we could take one of those fuselages and turn it into a flying airplane in 22 days. With 3 assembly lines, we were turning out 48 airplanes a month with plans to eventually reach 56 a month. After the shutdown and Covid, Boeing is now building 16 a month and they are struggling to maintain that rate.