I say give Boulder and Longmont the DMU train, but not until the state CDOT, injects it with some funds for a Front Range Commuter Rail Starter Program. This would mean the service would also extend north to Loveland and Fort Collins. Here's what I was thinking. Something along these lines. I split it into two lines, a North Front Range and a South Front Range line.
North Front Range would run like this:
Fort Collins > Loveland > Longmont > Boulder > Broomfield > Westminster > Union Station > Peoria/Smith Rd > Denver International Airport
South Front Range would run like this:
Pueblo > Colorado Springs > Mineral Station > Englewood > I-25/Broadway > Union Station > Peoria/Smith Rd > Denver International Airport
*Re-Route AmTrak SW Chief to Pueblo
SOUTH FRONT RANGE LINE:
-Front Range Commuter Rail using FRA-Compliant, 79-mph DMU's
-Run service mostly on existing Freight Rail Track, perhaps adding an additional track to the ROW where needed.
-In south Metro Denver run it up the Sante Fe freight line, parallel to the SW Light Rail Line.
-Have stations at Mineral, Englewood, I-25/Broadway & Union Station, all of which will be transfer stations with existing Light Rail Stations.
-Run on the CML into Union Station, with a new platform branching off between the CML tracks and the Light Rail platforms, where it can drop off and pick up passengers transferring between Light Rail and Buses.
-The train then loops around the Prospect District to the Commuter Rail lines so it can pull back into Union Station at the Commuter Rail Terminal, where it can pick up and drop off passengers transferring between RTD's Commuter Rail/AmTrak services.
-From here, this line would run on the East Corridor EMU line to Denver International Airport, for it's end-of-line.
NORTH FRONT RANGE LINE:
-Front Range Commuter Rail using FRA-Compliant, 79-mph DMU's (same as south line)
-Run service mostly on existing Freight Rail Track and RTD's NW Rail line.
-Start route at Fort Collins at a shared transfer station with their planned BRT line. Also have stations in Loveland and Longmont.
-The Longmont Station would be a shared transfer station with RTD's planned NW Rail line. The North Front Range line would run on the RTD NW track the rest of the way into Union Station, sharing all of RTD's infrastructure from here on south.
-Additional stops located at RTD NW Rail Stations in Boulder Transit Village, Louisville, Flatiron/96th Street-Broomfield, 71st Ave-Westminster & Denver Union Station (DUS) Commuter Rail Terminal.
-From DUS, the North Front Range Rail line would run on RTD's East Corridor EMU line to Denver International Airport, for it's end-of-line.
CONCLUSION:
I think this model presents a starter Front Range system which is both economical and practical for a service launch in only a few short years from the date of secured funding. One major point of emphasis, is that this plan would allow for all front Range commuter rail passengers to have one-seat rides to both Denver Union Station and Denver International Airport. the RMRA report clearly identified this one-seat service to Denver International Airport as a major ridership amplifier, over any setup which would require passengers to have to make a transfer to get to the airport. I think the one-seat service to DUS, will also serve as a ridership amplifier, for the exact same reasons. My proposed alignment accommodated both these ridership amplifiers and also has stops in the urbanized centers of the major cities along the route, as opposed to along the interstate highway system. Such an alignment would not rule out future track upgrades in at least key sections between urban areas, to allow for up to 110-mph service. These upgrades could be made in phases, as funding permits.
INTEGRATION WITH DUS: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ↑ To Ft Collins . . . . . .. To DIA →

. . . . ↓ To Colorado Springs-Pueblo
GREEN = South Front Range Commuter Rail
Blue = North Front Range Commuter Rail
RED = RTD Light Rail