I was under the impression that the Trek Alpha was sort of a low end model of Trek's aluminum bikes. I have a 1.5 and I believe the last model bearing the Alpha label was a 2.3. But as far as I've been able to discover, all of Trek's aluminum frramesets are Alpha. It has to do with the hydro-forming of the tubes and the aluminum welding technique that doesn't show the welds.

I installed a light set of wheels, a light seatpost and an integrated bar/stem with an 80 cm reach and the bike without seatpack or water bottle dropped from 19 lbs to 17.5.

As a comparison, this was the weight of my 2018 carbon fiber Trek Emonda with a 10 speed Record group. I also have a Douglas Vector aluminum with a carbon fiber rear triangle bike that weighs in at 16.5 lbs and I sold my Look that weighed 16 lbs.

I think that 18 lbs is about the cut off for me for "light". Heavier than this and I can tell on a long climb. But any lighter than 18 lbs and I can't..

From the feel of it the Trek Alpha rides LOTS better than the carbon fiber framesets. The aluminum fork on mine is stiff enough that I can't feel any give in it. Carbon forks are pretty stiff as well as long as they weren't built by Colnago which has a sharply tapered blade cross-section. But most carbon fiber forks are really stiff.

All in all I'm quite satisfied with a complete metal bike. Though the half and half Douglas Vector is plenty stiff and uses the 25 mm tubeless tires for suspension and that works very well. The 25 mm tubeless has the same give as the 28 mm clinchers.