A major extension is in the works for Toronto's shortest subway line, the Line 4 Sheppard.

Often derided as a 'stubway,' the truncated rapid transit route has been operational for over 20 years, and is known for some of the lowest ridership figures on the TTC network — a recurring statistic that could be attributed to its limited reach through North York.

Two decades into its operation, a long-discussed extension for the Sheppard subway could be just around the corner.

A page detailing a proposed Sheppard Extension was recently added to the Metrolinx website, explaining that "Metrolinx is currently studying options and seeking community input to extend rapid transit along Sheppard Avenue both east and west."

The transit agency states that "The Sheppard Extension would improve transit connections in Toronto's north end and make it easier and faster for people to get around Toronto and the GTA."

A map of the Sheppard Extension includes potential extensions from the current terminal stations of Sheppard-Yonge in the west and Don Mills in the east.

The current five-stop subway line would potentially see a westerly extension to Sheppard West Station and an easterly extension all the way out to Sheppard-McCowan Station on the upcoming Scarborough Subway Extension project.

Along the way, it would connect with up to three GO stations, giving the line multimodal connections that would further feed ridership.

The map also notes a potential future Sheppard East LRT that would follow the arterial from the subway's proposed eastern terminus deep into northern Scarborough, all the way out to Meadowvale Road.

Metrolinx's Sheppard Extension is currently in the initial stage of the transit agency's project planning and development process, known as the initial business case (IBC).

This IBC process reviews a potential project to determine how it addresses a problem and/or opportunity, as well as selecting a preferred option for further refinement and design.

Such an extension would close a long-envisioned loop in the TTC's subway network, linking the western branch of the Line 1 subway with the extended Line 2's future terminus in Scarborough, more than doubling the length of Line 4 and offering the potential to vastly increase its comparably low ridership.

A Line 4 extension would mean changes to existing terminal stations, including the potential activation of the long-mothballed centre platform at Sheppard-Yonge station.