Another great dream/idea: building a global railway uniting all world continents.
Of course,it's building would cost a lot and require combined efforts of many countries. First each country would build a fragment of railway crossing its territory, then these fragments would be linked together. The next stage would be constructing long tunnels/tubes bridging the continents over the oceans.
This would allow people to travel all across the world paying less than for airflights, though however fast such a transcontinental train would drive, it would take several days for it to cross any ocean or continent.

How do you think, are such railways really worth building? What are the pros and cons of this idea?