Frankly, your mention of dual-mode FLIRTs just made me like the idea even more!
Apparently, the FLIRT DEMU design is easily upgradable; the folks in Ottawa, who purchased the same design as TexRail, say their diesel units can be upgraded to electric catenary in the future:
• Video Link
I think it is important to look at what Frontrunner is becoming, and how the public actually uses it. To be honest, Frontrunner was never really a commuter rail line, not by the typical USA conventions. It had its own track, it ran all day, and it's ridership was much more diverse than the typical 9-5 crowd. The only time Frontrunner was a typical commuter train was between Ogden and Pleasantview, which proved to be more trouble than it was worth.
Currently, I would classify it as a regional rail line.
Some serious changes are in the works. UTA keeps adding infill stations, some planned like Vineyard and Draper, and others not originally planned, like North Temple, the Point, Ogden Business Park, and Centerville. These extra stops, plus the doubling of frequency to 15 minutes, fundamentally change the nature of the operation.
Using old beater diesel locomotives and long distance bilevel cars for what Frontrunner is morphing into would be kind of like using a semi truck as an Amazon delivery van. There are better tools for the job.
I think we can all agree on that point. The ultimate goal is fast, lightweight, electric trains designed for high frequency. The question is how do we get there most quickly?
This whole 'strategic double track' initiative proves to me that the decision makers at UTA and in the legislature don't care about the fundamentals - only results. By doubling the amount of trains on a line that is still mostly single track, they are doubling the potential for 1 of the twenty trains to throw everything off schedule. Don't be surprised when reliability plummets, and people start to lose trust in UTA all over again. Ultimately, taxpayers will spend more money upgrading the line multiple times (putting in switches in odd places and then tearing them out a few years later) than if they had just set the goal of full double track from the beginning.
But I accept that this strategy is based on the politics of the situation. It is what it is.
But do we think that the politics of the situation will change when it comes time to electrify the line? If we could not get one large project to double track the line in a single effort, what are the chances we will get full electrification and a complete fleet replacement all at once?
Basically, I don't think electrification is feasible to do all at once in the political environment. Switching to stadler equipment all at once is probably also infeasible. Our legislature works in many small strokes, which can be frustrating in some respects, but it also is a very stable way to govern. We need to update our strategy accordingly.
Let's say UTA does buy stadler DEMU FLIRTs for expansion. For the next few years, UTA would run both old and new equipment, just the way they do on TRAX. Through additional grants and funding allocations, UTA could continue to buy more Stadler equipment (which would be popular since it would be helping a local Utah business). The current Frontrunner locomotives and cars could sell for a decent price to other commuter rail lines, further offsetting the cost of new equipment.
Finally, after the entire fleet has been replaced, UTA can start a project of "Strategic Electrification," where short sections of track will get catenary- and hopefully full double track if it doesn't have it already.
The best part about 'Strategic Electrification' would be that all the Stadler equipment will be ready to go electric already - just add pantographs! No new expensive fleet replacement would be needed, because it already happened back when the fleet was small enough to do it in small, manageable chunks.
And of course, there will have been other benefits too, such as 15 minute service all day, longer service hours, and even Sunday service, since the FLIRTs are so much cheaper to operate than locomotives and coaches.
I'm still struggling to see any downsides to this approach.