Vilvoh have posted here: http://forum.simutrans.com/index.php?topic=1304.msg12661#msg12661 very nice screenshots as only one picture.
Give it a tool to make so big shoots?
It was not made by him.
IIRC you can set Simutrans to run in window, make this window very big and then make a mega-screenshot with the game...
You can use -screensize 3000x2000 or such in commandline. Although you should try it with nothing else running since it will slow down your CPU reasonably.
AFAIK, a person made a plugin for Freetrain to save a whole map screenshot.
Freetrain is written with C# and using Windows scroll bar, so capture software like "Faststone Capture" can do take a whole map screenshot.
In theory, it's possible because somebody (kaja47) did it in the past. see here (http://archive.forum.simutrans.com/topic/04383.0/index.html). In the blog article, he gave more details. He started making 1024x768, and then he joined all together using Photoshop. I don't know if it worth such an effort....
The complete article translated from cz to en here (http://translate.google.com/translate?prev=hp&hl=es&u=http%3A%2F%2Fk47.cz%2Fprojekty%2Fnejvetsi-screenshot-na-svete&sl=cs&tl=en).