Because they would hit each other and many people would die...
Yes, trains are allowed to cross international borders as long as the countries involved agree. Amtrak regularly crosses between the United States and Canada, and trains cross regularly from Pakistan to India and back. Many trains cross borders to other countries in Europe and other places around the world.
Eurostar other than the British Class 373s trains is also allowed to cross the border between France and England.
No, parallel actually means that the lines will never touch or cross
TGV is the name used by SNCF (French Railways) for their high speed trains. High speed trains which cross into other countries are run jointly with SNCF and other railway companies. They are some times branded as 'TGV', but sometimes other names are used. - Trains from France to Brussels, the Netherlands and northern Germany (Cologne) via Brussels are called 'Thalys'. - Trains to London, and London to Belgium are called 'Eurostar'. - Trains to southern Germany (Frankfurt etc.) are provided by both railway companies, and are either called TGV or ICE for the German trains.
The NEVER cross each other
same as any other object, gravity never takes a holiday! the 'motion' pattern is in the shape of a parabola.
If none of the sides ever cross each other, there cannot be a closed plane figure.
I'm not sure if you mean control the trains that only the computer can drive, (like the 30000 EXE, royal scot or empire builder) cause you can never do that. (to my knowledge) In explore mode, there are no other trains; it's like I am legend. But of course the signals still work.
The answer would be parallel lines these lines never meet or cross each other.
parallel lines
No, parallel lines never meet or cross each other.
No, the other trains in Britain and Scotland cannot travel faster than the trains in the high speed one.