US wireless carrier AT&T is headed below ground with the latest leg of its expansion in Mexico, building out a new fibre network to supply LTE connectivity in Mexico City’s Metro underground rail system.

Kelly King, the operator’s Mexico CEO, said in a blog post work is already underway. The ultimate goal is for AT&T to build a fibre-fed network to supply LTE and Wi-Fi connectivity across all of Mexico City’s 195 Metro rail stations and through 125 miles of tunnels.

AT&T will prioritise three lines Kelly said pose the greatest challenges due to their depth and high level of use by commuters.

The move comes as part of a broader effort from AT&T to push deeper into Mexico. AT&T is working to build itself up as a competitor for dominant Mexico operators Telefonica and America Movil following its acquisition of smaller providers Iusacell and Nextel Mexico in 2015.

At the time, AT&T committed to spend $3 billion to expand its wireless coverage to 100 million in the country by the close of 2018.

AT&T’s efforts extend beyond mobile phone service: the operator is also pushing to complete deployment of a new LTE-M IoT network in the country by the end of this year. In May, it completed rollout of the technology in the US, stating at the time its combined US and Mexico LTE-M networks will cover some 400 million people.