The current beta is 2.7 located at:
http://www.gigasize.com/get.php?d=8hf5r202mpd
IMPORTANT:
You will need to unzip the file,
copy and paste the file in the ROUTE folder of the sim,
Open the Activities folder and double click on the three apk files to install the activities. This will install the consist, path, services, and traffic files. They are required to be there for the route to show up in the route selection window of the game.
You will also need to download the required rolling stock to get the activities to work.
If you haven't used the game in a while or are just installing it for the first time you will need to install the game from the two CDs in the box.
You will also have to download and install a few other programs/files, all are free but without them you will get errors.
1 Goto the following and scroll down to the bottom of the page where you will find three downloads. You'll need the first to get the game to work, the third for North American equipment to work. The second is optional if you think you might want to use British Rail routes and equipment.
http://tsinsider.com/Support/v1Downloads.aspx#tsupdate
2 Goto the following and get BIN, this will improve a few things in the general operation of trains and also allow you to switch cabs in some activities.
http://mstsbin.uktrainsim.com/eng/eng.html
3 Goto the following and enter newroads.zip in the file name search,
http://www.trainsim.com/file.php?cm=SEARCH
4 Go back to the same search engine at Trainsim.com and enter xtracks.zip in the file name search.
That should about do it.
Paul

ADDITIONALLY
You will need...
Goto:
http://www.trainsim.com/file.php?cm=SEARCH
.... enter in the "File name" dialog box:
tsunpack.zip
Follow it's instructions to correct the version of the same file name provided by MSTS.
Unzip this file and place the WAG file in the "pchopper" folder at:
C:\Program Files\Microsoft Games\Train Simulator\TRAINS\TRAINSET\pchopper
This will give you two WAG files in the same folder, one for a loaded car, the other for an empty car (that I used for the activities).