What do you mean by "their track"? Any track on the board can be used or upgraded by any player, with two exceptions:
1. Colored surface tracks printed on all four edges of the board cannot be upgraded.
2. Tracks underneath a cart cannot be upgraded.

Were you trying to upgrade a track under a cart?

Hi Turtblu. Thanks for your comment. This gives me a better idea what's happening on the reported bug as well.

Was the T track rotated to the correct configuration on your action palette before you tried to place it? If the straight track you were trying to upgrade was vertical and T track was in it's starting position (exits to the East, West, and South), you would need to rotate the track tile on the action palette first, since an upgrade can only add one exit, and cannot remove any exits.

On the next release, I will add an error message that explains this when you try to perform an illegal upgrade.

There is an alternative interface for upgrading tracks by clicking directly on the exits you would like to add. You can try this by going to your user preferences for Off the Rails (while in game, go to the hamburger menu in the upper right or the options tab below the game board), and change Track Upgrade Method to "Add Exits"

I am certain that there wasn't actually a bug, but rather that the interface was not clear regarding rotating T-junctions on the action panel before placing tracks (see previous comment). I have added error messages and tooltips to clarify when attempting to make an illegal track upgrade (removing exits or adding more than one exit to a track).