I noticed the same thing in Edmonton. My grandparents live next to a major transit terminal in Castledown and my grandmother sometimes grumbles about "all the homeless people at the bus station". I always used to chalk it up to her being a bit fussy and dismissive, but sure enough, I was there last spring and pretty much 24/7 there seemed to be about 10-15 people sleeping on the floor and/or preparing needle drugs in the (honestly very nice looking) transit terminal. And that behaviour does tend to spill onto the transit vehicles themselves.
I couldn't help but think of the extremely similar Lacewood Terminal in the extremely similar Clayton Park area of Halifax, and how we don't see that there - yet. Occasionally there will be someone passed out in a bus shelter or on a bus, but it's the kind of thing I see maybe once a month, as a pretty regular transit user. And it's never at the larger, "indoor" terminals.
What we do see more and more here, in addition to all of the tents in parks*, is homeless/mentally ill/addicted people hanging out in the Central Library. At any given time maybe 5% of the people there are homeless, exhausted, not-showered-in-weeks, hacking up lungs, throwing up, passing out, etc. More often than not it smells pretty strongly of piss, and I've seen paramedics come pick up unconscious people a few times so far this month.
Basically these are the spaces that are available to them to get out of the weather, or to get some sleep somewhere where someone else isn't going to steal all of their possessions (if they have any). Or recharge electronics if they have any (most of these people don't seem to). The shelters here are all full, and I'm guessing it's the same story in Edmonton and Winnipeg. I think it's mostly incidental differences in climate and layout that result in homeless people taking up quasi-residence in the transit system in Edm/Wpg, vs the Library in Halifax. I noticed in Edmonton in particular - for most of the year, people aren't outside unless they really have to be, and even the homeless want to be out of the elements somewhere. It feels cruel to kick them out of the only heated/air conditioned spaces that they're "allowed" in. In Halifax, it's a bit more viable (though not 100% comfortable or healthy) for people to just be outside most of the time, and I'm guessing Halifax Transit's internal policy on these things is harsher than the western cities'. They definitely wouldn't let people get away with being in terminals overnight - there are perfectly good parks for people to sleep in
The biggest difference with the Library situation is that yes it would be nicer if everyone there was showered and not hacking up lungs or passed out pissing themselves, but life just sort of goes on around them for the hundreds of other people there, and if anything goes wrong there are 10 concerned bystanders for every homeless/seriously ill person, plus all of the staff. Whereas in Edmonton (and I'd guess in Winnipeg) the homeless people are very often the "dominant group" when walking into any major transit terminal, and probably even some vehicles. This can be pretty intimidating, and realistically I wouldn't be comfortable using transit more than a couple times a year if that's what it's like.
*This year the City designated 4 approved parks for homeless people to camp in, luckily it's been a warm winter. Mental illness/addiction/"haggardness" tend to be less pronounced with the people in the parks than the people in the Library, who I suppose sleep in the Library during the day because there's nowhere for them to sleep at night. Most of the "Library People" wouldn't be capable of setting up a tent, and wouldn't know where to get one, and aren't aware that's an option. The "Park People" are usually coherent and many of them have day jobs etc, there literally is just nowhere else for them to live right now. I guess it's possible that there's a lot of overlap and that the Park People just aren't noticeable in the library because they look normal and healthy (they would just look like miscellaneous strangers, not "visibly homeless/seriously ill")