I meant LRT systems, my mistake.
The Yonge Subway runs in a wide open cut. I would not consider that to be a trench. This is what I consider a trench:
This is the Alameda Corridor Rail Trench in L.A.
Personally, I consider certain types of open cuts to be trenches. If it is open to the air and depressed from the surrounding grades on both sides for the purposes of grade separation, it's a trench. The only portion of the open section of the Yonge line that I don't consider a trench is the section across from the cemetery, between St. Clair and Davisville. I don't think this qualifies as a trench because the grade of the tracks on the east side is actually higher than the grade of the adjacent street for the most part. Really the main difference between what you would consider and open cut and a trench is the fact that the side walls are vertical and concrete vs angled and grassed. They both involve excavation, and they both involve building bridges over the corridor.
If the land is available, why would a city spend the money to excavate the trench, build the walls, bridges, and other necessary infrastructure? Makes little sense to me especially if you're going to cover it up again. It's essentially cut and cover, and tunneling has advanced enough where it's not that much more expensive than C & C.
It is cut, but without the cover. And as I mentioned earlier, there are a lot of extra costs associated with the cover part that you don't get with the cut part (enhanced fire protection and exits, ventilation, etc).
Just tunnel under the land with a TBM, and sell the land to developers if inclined.
If the revenue generated from selling the land is greater than the difference in cost between trenching and tunnelling, then yes. But unless you're doing this through land with extremely high property values and the opportunity for significant density, I doubt that would be the case. On Eglinton West, you're not going to get your money back by selling the land to develop mid-rise apartments with the occasional 30 storey condo.
I can understand covering over parts of existing trenches and open cuts, but even then you may not use all the available airspace. Looking at the above picture, there is no way a trench is "marginally cheaper" than just leveling the land, laying the tracks, and associated systems. Cost is definitely a reason for avoiding trenching, especially if the land is available.
In the scenario you describe, no. But again, the choice isn't between doing a trench and levelling the land, laying the tracks, etc. It's between digging a trench and completely ripping up and redesigning a major arterial roadway. If the choice is between at-grade through the Richview corridor and trenched through the Richview corridor, then of course the at-grade would be cheaper.
The Yonge Line was not in a trench, it was a open cut. A wide open cut. The gradient north of Bloor cuts through the ancient shoreline, so it was either tunnel, or save money and keep the line in open cut. But it's not a trench. The Ottawa Transitway between Bayview and Tunney's Pasture is a trench. That's for certain.
Boils down to definitions in that case, and I explained my rationale for it above. But yes, the Transitway between Bayview and the Ottawa River Parkway (and between U of O and the Rideau River btw) is definitely a trench, which is being upgraded to be an LRT trench. It has also seen the tallest residential building in Ottawa spring up directly beside it.
I should point out, I am not against trenching as means of grade-separation where appropriate. But there is absolutely no way it is marginally more expensive than surfacerail and siginificantly cheaper than tunneling. It's simply cut and cover without the roof, and Cut and Cover is still pretty expensive. And disruptive.
Again, it depends on the scenario you're comparing it to. If it's trench in empty field vs at-grade in empty field, you're right. But if it's trench in empty field vs at-grade through the middle of an arterial that will need to be completely redesigned, that may be a different story. It won't be equal, but it may be close enough to warrant it as a serious alternative, especially when you factor in the plus of complete grade separation.
And with regards to disruption, which is more disruptive: trenching through a green corridor, and causing disruption at cross streets, or ripping up/redesigning an entire major arterial roadway? We all saw the kind of havoc that happened on St. Clair during that upgrade.
And for what it's worth, I'm not opposed to doing at-grade through the Richview corridor either, I just don't think it's necessary to rip up and rebuild an entire avenue when there's a perfectly good transit corridor sitting right beside it. An at-grade LRT or BRT route through those lands would be fine with me as well.