Thanks for your answer.
So, to be sure that I've just well understand the placement rules,you have to place your own locomotive near locomotives already places in others turn but not in my active turn!
For instance, I can't creat in the same turn a 3 section road by myself because I can place only a locomotive near the two stations but I can't place the last locomotive in the middle of these two first.
That is correct?
The way to think about it is you MUST place a train next to a city or another train. So to use your example, you could possibly create a three section road by yourself as long as you place the trains starting from the city and working across the board.
Using your example, you would not be able to place your first train in the middle because it is not next to a previously placed train and not next to a city.
Hope that helps..now if somebody would answer my question about "strikes"