Thread by @ChittiMarco

One of the various criticism against the REM is the fact that it will take over the Mont-Royal tunnel, the only route that allows through running via the Gare Centrale, notably for ViaRail High Frequency Train.

I think this criticism is not completely fair. I'll tell you why.

A premise: I've criticized the REM project at length on what I think are several shortcomings in the planning and implementation phases, but I don't want to be hostile to the project aprioristically and just relate taken-for-granted opinions.

Let's imagine that Montréal, instead of doing the REM, took the RER GO approach: transform its infrequent commuter rail network in a modern, electrified, frequent S-Bahn. Let's just forget for a second all the issues with CP,CN, the bridges etc.

Let's also imagine that a future RER-MTL network would also include the south shore branch of the REM and the one to the airport and the West Island with similar alignements. Let's imagine that all those branches use the M-R tunnel as their "stammstrecke", their trunk section

A system like that would have 5 branches on the North and 4 on the South and West, assuming Candiac and Vaudreuil line could be routed through the Gare Centrale (which is not so simple, but let's imagine it is, using the CN tracks through St-Henri).

Such a system can be arranged in a 5-lines service with all the lines sharing the central "stammstrecke", the trunk section of 9.5 km, going from Griffintown to Côte-des-Liesse with 5 intermediate stations, 3 of which connects with the métro

Let's imagine that you want a 15' service at peak, or even all-day, in all your lines. That is 4 trains/h * 5 lines -> 20 trains/h*dir on the trunk section. This is not saturation at all, but it's not really a situation in which you can fill long distance trains. Why?

In the southern approach there is no problem: Via Rail can have it's own tracks and use the terminal platforms for trains from Ontario terminating at Gare Centrale.

For Trains continuing to or coming from Québec there should be some flyover arrangement to the South that allows them to stop at platforms on both sides of the RER ones so to avoid conflicts at the tunnel entrance. Complex but possible? Let's say it is.

But the problem, with a 9.5 km, 2-tracks trunk section shared between 20 RER at 3' interval and, let's say 2-3 ViaRail to Québec, is that the ViaRail trains will have to queue behind the RER ones between Gare Centrale and C-D-L, that is 8km and 5 stations

The problem is that those will be closely spaced stations and busy ones, at least Mc-Gill and Édouard-Montpetit, with stopping times of 40 to 60 seconds and avg speed on the 30-35km/h, not very compatible with long or medium distance trains.

The fact that no city with a functional regional rail network has a "stammstreck" shared between suburban trains and the various medium-to-intercity services should ring a bell about the actual possibility to have extensive shared infras for RER/ViaRail.

Especially if you consider that Long distance trains schedules tend to be more randomly perturbated and to accumulate longer delays (imagine a Windsor-Québec 120' late), that can completely mess up the tightly planned schedules of the frequent S-Bahn/RER.

The argument for a shared tunnel exists in case of a scaled back, minimal suburban service where only the D-M + Mascouche is improved with better service but still in the 10'-15' interval max in the tunnel.

That said, the argument for a fully separate suburban network (like the RER, which is fully segregated from
mainline rail) but using automated metro technology looks less weird, if the REM can take over existing commuter corridors corridors (regardless of the CQDP control).

Again, which one is the wiser choice should be tested on a long-term, region-wide planning exercice that evaluates the various hypothesis in terms of feasibility, cost/benefits, etc.

For example, it might be wise to evaluate a second cross-city route not for ViaRail only, whose utility might be questionable, but for a more xetensive medium-distance intercity service to serve the growing centers outside of the CMM, like Joliette, S-J-Richelieu, etc.

Again, it's a planning problem in the larger sense of devising scenarios and studying them thoroughly, without prejudices and knowing what it takes, technically, politically and financially, to get there.