Depends on the consortium. The one I mentioned is walkable (it’s a mile), or free bus-able if one doesn’t want to walk, and it’s pretty typical for students to take classes at the other schools. Again, we’re talking about servicing a small handful of students (math majors who want PhDs) , but if they want to make it happen it’s not that hard.
That is not true of all consortia, of course. Bryn Mawr to Penn would definitely be more challenging.
I’m guessing any of the Claremonts to Mudd would also be pretty easy.
I found her posts in the 2013 thread, finding I disagree with some of what she says though.
Seems the grads from the Williams link above were mostly doing what she says the MIT grads were doing, for example.