Each version of Trainz (Well, TRS04, TRS06, and TC1/2 anyway, not sure about earlier versions) had these sorts of things:
TRS04 has (that I know of):
alt + MMMG - randomly places trees, textures and hills/pits on the empty map
alt + fly - allows you to 'fly' around the map, and view from practically any angle
alt + walk - allows you to move around at 'head' height around the map
alt + smite - removes the menus, and allows you to 'destroy' objects/areas of the map. Click on an object, and a lighting bolt shoots down. The object is then deleted and a burnt looking texture is placed where the object was.
TRS06 (that I know of):
alt + y - replaces the alt + fly. Exactly the same otherwise.
alt + u - replaces alt + walk. Basically the same otherwise.
TC1/2:
no idea on what they are in this version.
TC3 (that I know of):
alt + y - same as TRS06
alt + u - same as TRS06
ctrl + alt + a - same as alt + smite in TRS04. Has an added smoke effect now though...
shift + ctrl + alt + t - click on a car after
shift + ctrl + alt + p - replaes 'alt + MMMG' from TRS04. Does the same thing.
Probably more easter eggs in all of the above

Zec