Generally I think concerns about gentrification displacement are overblown. Not saying it never happens, just that there is much less involuntary displacement in most cases than people, who often invoke it for other purposes, claim. Still the one worry is that poorer people get priced out of areas with good transit. That's not about displacement per se, it's about a lack of neighborhoods with transit access being available to people who really need, as opposed to just want, it.
(Regarding displacement: neighborhoods have a normal amount of population churn, and some poor people actually own their houses. Involuntary gentrification-displacement is really about existing residents being priced out due to rising rents or property taxes, not simply people choosing to move and then their replacement being a bit richer.)