http://support.microsoft.com/kb/302480
Train world initialization is when MSTS is actually building the train route file.
Usually you get this when something is missing from the route, either an object or a piece of rolling stock. The fastest way to fix it is to run Route-Riter (free) to check the route. It goes through the route a piece at a time and looks for things that are missing. It often will replace missing route objects right away with either the same thing or something similar. It can also generate a list of missing rolling stock and will allow you to pick a substitute. The most common problem is that something is spelled with a capital letter in one place and a lower case letter in the other, which MSTS does not like (Route-Riter fixes these problems too). It will also pick up objects which were supposed to be part of the bat file but somehow did not get transferred. This will fix it 99% of the time.
http://members.optusnet.com.au/virtualtrains/