CDPQ Infra prepared a FAQ on this and made it available as a press release:

Frequently asked questions about REM de l’Est’s underground portions

Why, after saying for months that an underground route was impossible, are you now announcing a tunnel section?

As part of the detailed planning of the project, two studies were conducted to evaluate different route scenarios. The underground scenarios were deemed inviable due to the many critical risks that accumulated. However, an alternative was identified more recently in a second study. This alternative involves burying a portion of the route in a surface tunnel between Jeanne-Mance Street and Robert-Bourassa Boulevard. This optimization has been incorporated into the reference project and will improve not only project integration, but ultimately REM de l’Est’s frequency by adding a tail track. The option of burying the entire downtown route is still not a viable option; that is what we have stated in the past. We are open to improving the reference project presented in December and will continue to do so for the rest of the route. 

How do you explain the fact that there are underground networks elsewhere in the world, but that it is not possible here?

Each city has its own characteristics that have to be carefully analyzed in its own context. Soil characteristics in downtown Montreal are unique. The glacial origin of the soil, combined with the water table level, make excavation particularly complex for this type of soil. For example, the risk of a TBM breaking when it drills in this type of soil (composed of rock, soil and boulders) is extremely high. We should also remember that we would have to deal with the metro's yellow and orange lines and the City of Montreal’s very old water and sewer lines. 

For a more comprehensive picture of the special characteristics of the downtown Montréal underground, please listen the explanations presented during a technical briefing held on September 2.

Was an elevated route selected for REM de l’Est because of cost? 

The optimization announced on September 2 brings the underground length of the route to 8.5 km, or roughly one quarter of the entire REM de l’Est route The studies published state that we took several scenarios into consideration for the downtown route. The options set aside were deemed inviable due to the high rate of critical technical risks (such as the impact of buried public infrastructure, the metro’s yellow and orange lines, the geology of the area, construction risks, high water table levels, etc.), not due to cost.

Ensuring the REM de l’Est integrates harmoniously into its surroundings is a top priority, as illustrated by the many initiatives planned to achieve this, such as establishing an Multidisciplinary Expert Committee and collaboration with the Québec-based architectural firm Lemay.

Why not move the route further north of René-Lévesque since the geological conditions there are more favourable to an underground route?

In order to meet the needs of as many users as possible, the location of the route was determined through a series of ridership studies and analyses. Based on this, the route on René-Lévesque was identified as the best one to meet the needs. The goal of REM de l’Est project is to provide a reliable, fast and efficient transportation solution. 

What about the rest of the route in Montréal’s East End, beyond downtown, and the concerns raised?

As emphasized on June 21, the objective is to continue to optimize and improve the REM de l’Est project following the first phase of public consultations, which reached over 32,000 people. The technical briefing held on September 2 related solely to the downtown sector and the publication of technical studies conducted pertaining to the various underground scenarios evaluated. We will return in the fall to present the optimizations we are currently evaluating and the architectural vision defined by the Multidisciplinary Expert Committee and architectural firm Lemay. 

https://www.cdpqinfra.com/en/news/pressreleases/frequently-asked-questions-about-rem-de-lests-underground-portions