Would it be possible to mix the Hiawatha Line with different equipment? With the numbers growing like they are, I think, not only are they going to need to add seating capacity. But they are going to need to add departure, such as a late night departure. Might not be pretty, but whats wrong with using both sets? Horizon's for the peak times and the Talgo's for the midday/evening trains? That's assuming the scheduleing is even feasable.
I believe CP won't allow extra service until certain infrastructure improvements are made (exactly what those improvements are, and where we are in the process of getting them done, I don't know).
The problem with using mixed equipment is that you'd require three (or maybe four) trainsets to do the work of two. Since much of the maintenance work is time-based rather than mileage-based (though there is some of the latter), you're not saving very much by running some equipment less often. Then there's the fact that you'd have to pay for the Talgo operation and maintenance, as well as pay Amtrak for tying up a set of equipment. Costs will only rise, with no real benefit, under that scenario.
The trains currently run on a two-day cycle, for maintenance purposes. So, day 1 a train runs 329-332-333-336-337-340-341, and day 2 the train runs 330-331-334-335-338-339-342 (and the other set does the opposite).
The two peak trains are 330 and 339 (which see standees on a somewhat regular basis). So, in order to cover those trips with Horizons, you'd need a Horizon set to overnight in MKE (or run a really early morning trip up, which would be guaranteed to be empty; then there's the issue of crewing that early morning train).
If you ran a Horizon set up on 339 and had it sit overnight and come back in the morning as 330, then you'd have no equipment to run for 342. If the Horizon set on 339 turned for 342, then you either have no equipment for 330, or you need to run the Horizon set back up (the early morning train, or perhaps an extra evening departure, which wouldn't be a bad thing). But then your Talgo turns don't work.
I suppose you could do a 339-342 turn, and just make 330 run with normal equipment.
The other problem with converting 330 back to Horizon is, that's the MKE originator. That's the train that will be coming out of the Talgo maintenance shop in the morning (with trainsets alternating on an every-other-day cycle). It would be kinda dumb to have all your Talgos overnight in CHI, with the maintenance shop in MKE, while your Horizons, whose maintenance base is Chicago, overnight in MKE.
Let this be a lesson to you kids, this is what happens when politicians make decisions. You have two expensive but marginally useful trainsets being built, and they still don't know where they're going to be maintained, or even whether they'll enter service, but they're almost ready for testing in the near future.
It's sorta like the Turbo Train fiasco in New York.