Tomorrowland Transit Authority at Disney's Magic Kingdom is set to reopen tomorrow, September 1.
The ride has been closed since April 19, primarily due to renovation of the Space Mountain rollercoaster, one of several rides through which the Transity Authority's course takes riders. Space Mountain is still due to reopen on November 22.
Themed as a "mass transit system of the future", Tomorrowland Transit Authority carries guests on a train ride through many of Tomorrowland's other attractions. It was originally scheduled to re-open on August 25, suggesting that the refurbishment work is not progressing exactly to plan but is not significantly delayed.
You can keep up-to-date with the latest confirmed and rumored ride "rehab" periods using Theme Park Tourist's "Rehab Watch".