Just a heads up, int'l-to-transborder at YUL was closed and unavailable at ~3:00 p.m. this past Wednesday. Wife & I came inbound on QR from DOH at 2:40 (delayed until 2:55 because paramedics had to offload a pax), and had separate tickets to DTW on DL departing at 4:20 p.m. The upstairs door to transborder was closed, and no one at all was stationed upstairs above immigration to direct traffic as there sometimes is. We asked a couple of officers downstairs who didn't realize that it was closed... they asked an AC agent about escorting us through, but AC said they'd need to contact DL (LOL), to which DL said no (another LOL). So the officers said just go through immigration (after filling out the health declarations), then go back upstairs, check in, and go through US preclearance that way... thankfully the place was deserted due to covid or we wouldn't have made it!

The lady at immigration didn't have any problems with it (I have a Canadian passport but my wife only has US), she marked our health declarations as exempt from the quarantine orders (we listed "transit only" for the address). She asked why we didn't go through transborder, we said it was closed and no one would open it, she thought that was surprising, and said "well good luck!" and off we went.

Ended up at the DL gate by 3:25 with ample time to spare... (and there was an AC backup to EWR at 6:55pm in case we missed)... presumably on an AC-to-AC connection they would have the int'l-to-transborder connector open, but who knows.