Maybe others knew this but:
The track width seems to be controlled by the Map theme being used.
I found an online editor for the themes here: http://www.javawa.nl/zumocolors.html
Point it to your favorite theme and modify the setting for Item 46 (TRK). I made it 5. Make sure you change the details so you don't over write the original theme.
Here is the original UK Theme (TRK border set to 1):

And here it is with the track border set to 5.
