Testing has begun on the Luas Cross City, which will connect the Green and Red lines.
Work on the project started in June 2013, with the first passengers set to be carried on the extended line in December.
The construction on the new 5.9km route involved extending the Green line and creating an interchange with the Red line.
As part of today's "gauge run" tests, overhead wires above the track were powered on as Luas trams travelled at walking pace between St Stephen's Green, through O'Connell Street, to Broombridge in Cabra.
There will be 13 new stops, with eight of them in the core city centre area.
It is expected the Luas Cross City project will create between eight and ten million extra Luas trips annually, while reducing the annual number of private car journeys by one million.
Testing will continue until December, with driver training beginning in September.