Hi beneb
As you are fairly new to Trainz, I should perhaps explain what I meant by having all the dependencies on both computers.
Dependencies are all the things you place on a baseboard to build up your route - track, platforms, buildings, trees, people, etc. When you save a route all the information about it is placed in a map folder - but this only lists the dependencies and where they are on your baseboard, not the dependencies themselves.
If you use only built-in dependencies - just the track, buildings etc. - that were included in Trainz, and you and your grandad have the same version of Trainz on your computers, there is no problem - they are all there. If you use your computer at home to download anything from the Download Station and use it on your route, then that must also be on your grandad's computer - so he will either have to download it or you will have to move a copy from your computer to his. Otherwise when you try to run your route on his computer you will get a message saying that some of the dependencies are missing and you will be offered the chance to connect to the Download Station to download them.
Again I can only talk about TRS2004 and other versions may be different.
My apologies if this explanation is not necessary - no doubt you know more about computers at the age of 10 than I do at 79!
Ray