Welcome to Trainz.
First thing's first, you need to slow down a bit. The Trainz learning curve can be a steep one and you're best to take it a little at a time.
As far as I know, nobody has made a 378 to date and I don't know if anyone is doing any LO stock. Creating an asset such as an item of rolling stock takes a great deal of time. You could be talking many months to do just one carriage and bear in mind, most content creators do this for no form of reward other than their love of the sim and all things railway related.
Your best plan is to search the DLS for content already made. There are many thousands of items there you can download and use if you don't already have something built in. It takes ages just to trawl what comes with Trainz so it could be you already have many items you would like.
I've been messing with Trainz for a few years and have yet to make the leap into content creation. It's a matter of time rather than anything else. The alternative to content creation is reskinning existing stock. This involves, in essence, repainting carriages to suit your own needs. While not always prototypical, it can often fill a gap until something better may or may not come along.
Using a free image manipulation program like GIMP or paint.NET, you can create your own liveries. Just beware you don't make such items available publicly unless you have permission from the original creator.
There is a basic tutorial on reskinning here
http://en.wikibooks.org/wiki/Trainz/Basic_Reskinning_for_beginners which may give you a taste for reskinning.
Take a look at the UK Screenshots
http://forums.auran.com/trainz/showthread.php?t=315&page=670 and you'll get an idea just what can be done with existing assets Isome items you will see are custom created).
Hope this is of some help and you enjoy Trainz as much as I do.
PFX.
PS, it's just possible I
may be able to do a LO liveried EMU but it wouldn't be a 378. I need to make a few enquiries first. Leave it with me and I'll post again here if I can do it.
