The Disney Parks Blog calls the train stoppage temporary but gives no indication when it will start back up.
Beginning this December, the Disney World Railroad will cease operation and the train will remain parked at the Main Street Station for the duration.
On the plus side, the locomotive will be available for photo opportunities that aren’t normally an option for guests, so that will be the trade-off guests will need to make.
Construction on the Tron rollercoaster might only require the tracks to be blocked for a few weeks or a couple months, but depending on how much work needs to be done, it could actually stall the train much longer.