Let me start with this:  The tool that makes this process possible is Unleashed Editor 2 (UE2) from twisteddirt.  There is a lot of 
information to be found at their forums, check out www.twisteddirt.com.  This tutorial will outline the process to create tracks that 
cover multiple tiles.  

One first must understand how MvA tracks go together.  There is a 5x5 grid of tiles - so 25 total tiles.  The center tile, row 3, 
column 3, is the tile that the Rainbow Track Editor (RBW from here on) lets you change.  When making a national in RBW, the perimeter 
tile, which you can change the disp and tex, is then repeated for the remaining 24 tiles.

UE2 allows you to package a disp map and texture together - you can export tiles anywhere on your hard drive.  Likewise you can import 
tiles anywhere onto the 5x5 grid in UE2.  

Put these 2 facts together - 25 available tiles, and a way to export and import them.  If this dosen't strike you as a REALLY big deal, 
then don't bother reading on.

There are limitations though - I've heard, although not tested myself, that a total of 6 unique tiles are possible.  That means you 
can have 5 individual tiles, then the 6th one will need to be repeated (20 times to make a total of 25 tiles).

Another interesting fact is that you should be able to place your unique tiles anywhere - so instead of having a cross of 5 unique 
tiles (3 up, 3 across) you could format yourself a 2x2 square, or a 3x2 rectangle, or a 5x1 row, or anything else you could dream 
up - creatively using the repeated tile, you could link unique tiles together that aren't adjacent - I mean you could dream up a 
track that does cover all 25 tiles if you really utilize the repeated tile.

The possibilities are endless!

Understanding UE2 -

I found the transition to UE2 very difficult.  It's not at all like RBW - and is very intimidating.  This tutorial is meant to 
help readers thru the process - to make a difficult task much easier.

The first thing to understand is that UE2 reads and edits pak files, the actual track file.  RBW works with project files, then 
saves off a pak copy - it's a different approach.  

So what you need to do to use UE2 is:

Create a track in RBW, save the project and pak at the same time, then open UE2 and open the pak you just created.  You can
then edit to your hearts content in UE2, but you only save the pakand to the best of my knowledge you can't go back to RBW.  
But you wouldn't really want to.

Ok so what really makes a track?  It's a black and white disp map, a colored texture file, then objects placed on top (the locations 
of which are recorded in a wordpad-like file) and track activities (also a wordpad ish file).  Each tile you want to use for your 
track will need a disp map and texture, then you'll add objects and activities to the compiled tiles using UE2.  The method you use 
to make your disp and texture are beyond the scope of this tutorial - just make sure they match at the edges.

So lets start making our tiles - first start up RBW.  Very important - make sure to make the track a national and choose your scale, 
then set your disp strength in RBW - you'll need to decide the values you want to use up front and use them consistantly each time 
you build a new tile.  UE2 will read these values when you create your tiles.

Now that your scale and strength are setup, import your disp and textures into RBW - they don't have to be final, as they can be 
updated later - but lets assume you have a pretty good start.

Now save your track (the project file) in RBW and export the pak - make sure to name it something easy to remember, like "awesome 
new track tile 1".

Do the same thing (create an RBW project, and pak, with disp and textures, all set at the same strength and scale) for your other 
tiles ("awesome new track tile 2", "awesome new track tile 3", etc).  Now we are done with RBW until you want to update your tiles 
later on.  This sounds like a lot of work but it really goes fastall you are doing is 4 things in RBW then saving.

(PS I like to do all my disp map beta testing in RBW - you can load up 2 adjacent tiles at a time if needed - although you could do 
it in UE2.  I just think it's way faster to load up the tweaked disp map in RBW each time you make a lil change)

Lastly, use RBW to create a blank track, setting only the disp strength and scale you want to use.  Save the project and export this 
pak - maybe we'll call it "awesome new track".  This will be the file where we gather all the other tiles together.

Let me stop here, and make a blanket statement about what we are doing:

You create a pak from RBW with each individual tile in it.  You then use UE2 to export each tile.  You then use UE2 to combine all 
your tiles into one track in a different file.

So the next step is to open each "tile file" you've created in UE2 and export the tiles.  So in UE2 go to the open file button, 
and select either the project file or pak file for "awesome new track tile 1".  When it's open, go to the terrain tab on the upper 
right of the screen, select the center tile (notice how the 25 tiles are displayed) which should be your disp map and texture, right 
click, and export the tile.  Give it a good name and save it somewhere you can remember.

Do this process for each tile.  Open and export.

Now open up the blank file we created "awesome new track".  This is where the magic starts to happen.  Again go to the terrain tab.  
Now you can left click on any of the 25 tiles, right click, then IMPORT your tiles that you've saved.  Repeat for each tile location 
you want to use.  Gorgeous isn't it???

When you import your tiles, UE2 asks you if you want to just this one tile- this is a very important question.

If you answer Yes, it will only replace the tile you've selected.  This is probably what you want to answer for each unique tile.  

There are 2 times you will want to answer no.  The first is when you import whatever you want your repeated tile to be.

The second time, and this is really important, is when you go to update each tile.  If you do not click no, which replaces all 
similar tiles, UE2 leaves the old tile file in there, and puts a new one on top of this.  Eventually your file size will balloon, 
and once it reaches around 100 megs, it will crash and burn.

This is probably a good time to re-emphasize that in UE2, you need to save PAKs out - and when you reopen a project, also open the PAK.
Remember UE2 works with the PAK file, NOT the project file.

Lets talk about how you update your tiles once you've imported them for the first time.  The way I do it, and there are probably 
other ways to do it, is to open my RBW project file, update the disp and texture as required in RBW, save the file, export the pak.  
Then open the project in UE2, export the tile again (it's ok to overwrite the tile file you already created), then open my "awesome 
new project file" in UE2, and import the tile.  

Obviously this isn't a streamlined process, so I only update after I've made major changes.  You can, however, update your texture 
using UE2 simply by importing the file (under the terrain tab, then click on the texture checkbox on the bottom) - it's just that you 
can't update your disp map.

Another note, remember that disp is just a file - you can evolve your disp map all you want, using whatever photo editor you use, 
and do beta testing in RBW without saving anything - just set the strength, scale, and import the disp to test.

As for testing in UE2, one thing you have to do before you can try your track out, is add a freeride activity.  In RBW, the freeride
is there by default, but NOT in UE2.  To add it, just go to the activities tab, select freeride from the pulldown, and click the add 
button.  That's all there is to it.

Ok so we've covered how to import your disp and textures into UE2.  Now you can freeride on your multiple tile terrain.  The next step 
that is critical is to import a spline so you can turn your track into a "real" track.

UE2 can edit splines, quite well, but it cannot create them.  So we rely on RBW to create a spline, then import it to UE2, and edit it 
as required there.

IMO the best way to do your RBW spline is to create a perfectly scaled version of the final spline you want to use.  To do this, combine 
either your disp maps, or textures, or both, into one square file.  For instance, if your track will cover a 2 tile x 2 tile grid, you'll 
to combine all 4 pieces into a single file (assuming all pieces are 257 squares, you'll end up with a 514x514 file that you can import into
RBW).  If your track is 3x3, you'll have a 771x771 file to import into RBW.  Then you just place your grid the standard way in RBW - although
you should understand the track widths will be much smaller then what you are used to. 

Once your spline is all done in RBW, export it - remember where it is, then open your UE2 file, go to the activities tab, and create a national
activity.  Then go to the bottom area, the spline portion, and import the file you just made.  Using the botton that looks like a magnifying
glass, you need to scale up your spline to fit across your multiple tiles.  If your track covers 2 tiles (ie 2x2 grid), multiply it by 2, if
it covers 3 tiles (3x3 grid) then multiply it by 3, etc.  Once your multiplier is in the box, you just click the magnifying glass.

Then you have to move it to the center tile, so plug in (256 times your grid scale, which btw is in "mcm2" terms, not RBW terms, so 
you'll have to look in UE2 to see what it is - 3.5 is equivilent to RBW scale 6, so the number you use is 896) into the same box you 
just put your scale multiplier value.  You should see the spline appear on screen at this time.

You can edit nodes by RIGHT clicking on them.

You also will need to place the start and finish lines...by right clicking on a node and hitting the appropriate button.

This process is well documented on the twisteddirt forums.

So having your multi-tile terrain imported and placing a spline are the most important things you need to do - you also can place
trees, objects, and modify track settings in UE2, but I believe that is beyond the scope of this tutorial.

So, to summarize:

A creator can use UE2 to create tracks that cover multiple tiles.  The process works like this:

Use RBW to import a disp map and create a pak for each tile you want to use.

Use UE2 to export each individual tile you've created.

Use UE2 to IMPORT each tile into a single track.

Create a spline in RBW.  Use UE2 to import the spline, then scale and move it over the terrain you've already created.