What does this suppose to mean, you ask. Isn’t Relativity already extreme enough?
A proton in CERN can reach the weight of a mosquito, due to its near light speed. During the acceleration a lot of energy has been transformed into the proton’s relativistic mass, just as Einstein would have wanted.
Speed it up some more, and it will weigh as much as a mouse or even an elephant. Then if it were to be accelerated to a speed even closer to c it would became so heavy as to equal the weight of a mountain or even that of a continent! Approximately at this point the proton would become a black hole. Not a microscopic black hole which would allegedly soon cease to be, but a substantial one, radiating the Hawking radiation in megawatts. If you were to push it even more after that, the resulting black hole would cool down, and thus reach a state where its mass would surpass the mass of a planet, with a radius of just one meter.
Only a fraction of the energy of a supernova explosion is needed to transform a proton into a black hole, more massive than the Earth, and with a radius of just 1 meter. So I have no concerns about the feasibility of such an experiment.
What worries me is this: It’s a black hole for the stationary observer, but for the observer on the space ship chasing it, it’s just a humble proton which will likely soon acquire an electron, and become a very innocent hydrogen atom – the most common thing in the Universe.
So you see, black holes can indeed be relative! For one observer an object can appear as nothing more than a small rock, for another it can be a super massive black hole!
This could be called an extreme case derived from the Theory of Relativity. The relativistic contraction only makes a relative black hole more achievable.
If the Einsteinian train shortens in addition to gaining mass, it collapses into a black hole even faster. Only for the observer on the platform of course! To the observer on the train (inside the black hole) everything looks normal.