PA 125 does not directly access Interstate 81 (I-81); however, north of Pine Grove, there is an interchange with SR 3013 (Molleystown Road) just yards from the highway in Ravine.

PA 125 begins to run a short distance to the east of I-81 as it passes through a forested gap of Second Mountain and crosses the creek.

PA 125 becomes undivided again and runs through more forests parallel to the Swatara Creek, turning north-northwest as it passes through Echo Valley and crosses Sharp Mountain.

The route crosses a Reading Blue Mountain and Northern Railroad line prior to entering the borough of Tremont and heading northeast on Branch Street, passing through wooded areas of homes.

PA 125 heads back into Tremont Township and comes to an intersection with US 209, turning southwest to form a concurrency with that route.

The road continues through more of the community and becomes West Center Street before leaving Donaldson and heading into more forests, running a short distance to the north of the Reading Blue Mountain and Northern Railroad line.

The route runs north through the wooded residential community of Lamberson and continues into agricultural areas with a few homes.

[2][3] PA 125 crosses into Upper Mahanoy Township in Northumberland County and becomes an unnamed road, soon turning to the north in the community of Line Mountain.

PA 125 crosses the Mahanoy Creek and winds northeast through a valley, passing through areas of farmland and woodland.

The route heads into the commercial downtown and becomes North Market Street, crossing the Shamokin Valley Railroad and passing more businesses.