I followed the Denver - Boulder - Longmont existing line from where it branches off of the big railroad yard north of I-70.
It is single-track all the way, and it is an active railroad line. I looked at it with Google Maps, and I saw a train on it. I checked what I was doing with OpenRailwayMap, and that line is called the Front Range Subdivision, and given the color "Main Line".
The rolling stock will be diesel, either loco-hauled or DMU's. Extending the Denver-commuter electrification out to Boulder and Longmont will be too expensive to justify for a peak-time service, I'm sure. US commuter-service DMU's are mostly separated from other traffic, so they are unlikely. That leaves loco-hauled trains, and they might be too much for the amount of service. That may explain the factor of 17 extra subsidy that this line might need.
I checked on the midday frequencies of the existing commuter lines, and I found:
A: 15 min, B: 1 hr, G: 30 min, N: 30 min.