The DailyCamera reports of Four Mile Creek Extension in the works now with the city and the county. The extension will connect Four Mile Creek Trail with the Cottonwood trail, in the north-eastern part of town. The extension will pave the existing dirt trail that runs under the Diagonal Hwy and also create a tunnel under the railroad tracks. Work will start as early as November. The official Boulder County Project Plan for this project.
Four Mile Trail runs east/west (horizontal in picture) and the new extension starts right at the Diagonal Hwy (Hwy 119)
This isn't the only extension in the works, here is what is said in the article
Both the city and county have other trail-connection plans in the works as well, including an extension on the eastern end of the Wonderland Creek Trail, which would also create a formal crossing to the railroad tracks that lie just east of the Foothills Parkway near its intersection with Valmont. Like the Four Mile Creek Trail, a dirt-track social path already extends the Wonderland Creek Trail across the tracks.
This project is a final project from some federal money received years ago, one project already successfully completed was the intersection of Diagonal Hwy and Hwy 52. The South bound land now has a bike lane (I believe) through the intersection where as before you had to enter the lane to get through the intersection