State transportation officials have stuck to their promise that MBTA riders would be able to track Green Line trolleys online by the end of the year.

Starting Thursday, users will be able to see the locations of above-ground trolleys on their existing MBTA tracking apps, like they already can for the Red, Orange and Blue lines.

"[The Green Line] was the only system that we had where we didn't have any sort of location information," said Dominick Tribone, the T's special assistant for strategic initiatives. "The secretary of transportation said, 'This is ridiculous that we don't have this information on our second-busiest line.' "

To that end, the T spent a little under $1 million acquiring and installing GPS tracking devices — half the predicted cost.

But this is only the first phase of the roll-out, Tribone said. The T is still working on tracking trolleys that are underground and offering estimated arrival times.

"Predictions are fairly difficult to do," he said. "Locations are easy. So once we have the GPS locations, we can at least give those to customers while we work on getting the prediction algorithm more accurate."

Tribone expects that the T will be able to give its users arrival times by the end of the fall.

Tracking trolleys underground will require installing new sensor equipment. The installation is expected to be completed in early 2015 and available to customers some time after that.

Clarification: The real-time tracking data from the MBTA begins being offered at noon Thursday, not first-thing Thursday, according to a T statement.