Simplify this check by testing the rail vehicle info flags instead of passing a global variable around.
Note: This is not exactly the original behaviour, because the Lev4 was always allowed to be built, but i guess that was a glitch.
abilities you really don't want to know about (free bridges, etc..)
I removed this flag some revisions ago, but the Aircraft part
depends on it, so I re-enabled it again..
-Fix: removed the ability for the oldAI to cheat (this will criple him somewhat)
-Add: base-code for many improvements to come in the AI-system
-Add: added base-code for multiplayer AIs (DOES NOT WORK YET!)