I would see the Yellow line contining to Parc station under the avenue of the same name. Or to be cheap just a new extension to McGill as it was planned by the AMT years prior.
The Lachine line is expected to be REM in transit circles.
A stop near Du Parc and Des Pins would suffice, no need to parallel the Orange Line for the entire stretch to Parc Stn.
I will step a bit into fantasy territory here, be warned, probably because the situation in Montreal seems so hopeful and optimistic in contrast to Toronto's but here goes:
This would solve a number of issues currently facing Montreal:
- What to do with the Yellow Line. This has been a 3-stop stubway since the 1960s. To make the line more useful, have it extend south via Chambly to St Hubert Airport, creating a secondary air hub for the region after PETA with a direct link to downtown
- Gives the Pink Line viability. Valerie Plante used the Lachine link - which was incorporated after her initial Pink Line proposal which only went as far as Gare Centrale - as a marketing ploy to win votes in the western suburbs. Without this key component, the Pink Line loses some of its sheen but interwoven with the Yellow Line gives it back that broad, all-encompassing regional appeal
- Links Montreal Nord to the south shore and downtown. Commutes that take hours now could be a half hour or less per this plan.
- Doesn't create too many interchanges. With McGill and Bonaventure becoming interchanges, adding Place des Arts and Mont Royal into the mix would have been too much overkill. Dumping loads of passengers at random spots requiring more transfers is not a good idea. Berri UQAM Yellow Line platforms would suffice.
- Chance to modify tunnel in Old Port for a Bonsecours Station. Also potential interchange with the Notre Dame Tramway.
- In the inner city add another stop near La Fontaine Park. And in St Leonard at Lacordaire and Couture.
- More likely to get CAQ approval because it runs through Longueil/South Shore, not just Island of Montreal