One of the strategies I follow in Osaka map is one line using exclusively Shinkansen trains and the rest of lines using usual locos (as real life). But I can’t use this strategy in Extreme mode because usual trains are added automatically to a line when it is built.
The most simple way to explain this: Once the second week has been reached to get a Shinkansen-type train, I have to add the three usual locos to the first line, build the second line (empty) and finally add the Shinkansen to this second line. I have been forced to add all usual locos to existing lines before building the Shinkansen line.
My suggestion is not add automatically locomotives to lines in Extreme mode, at lesat in Osaka map.