The only reason I can guess that AMT is running 8 cars instead of 10 on the DM line is that they have a backlog of repairs and/or they ran out of spare parts.
Running trains over capacity for a small periods of time would not probably cause problems but it has been a number of years now that the rush hour trains are way over capacity.
The persons frequenting this discussion board seem to know what to do better than AMT .
As far as I can see (DM line) so far is that the East - West junction underpass should have been built already to be able to increase traffic.
All lines to be at least "GREEN" should have been electrified. Quebec produces no oil but a lot of electricity.
Common equipment to run on the electrified lines should have been ordered.
Heated storage and maintenance garages built to house the trains so they are well maintained and free from "ICE".
Just my thoughts, but I am not a high paid transportation executive so do not know very much.