They're implying that the trains would be filled with homeless people.
They tend to avoid shelters because of rules against drug and alcohol abuse, favoring instead tents or public transport such as Palo Alto's "Hotel 22" bus route:
https://www.cbsnews.com/sanfrancisco/news/documentary-hotel-...
Most cities rule out the stick unless the carrot is provided, which is correct. But a significant number of those on the side of handing out carrots don't want to tie it to the stick.
So homelessness is rampant and everyone is unhappy. Perfect.
Policies around behavior in public spaces are actual live debates.