A lot of people ride the shuttles between Boston to NY, and DC to NY. It's pretty convenient. Definitely a step down from traveling in the bubble when he held a cabinet position, but not entirely abnormal.
He was at Yale hosting an event with HRC, Condoleeza Rice, and Madeleine Albright.
https://news.yale.edu/2019/04/18/albright-rice-clinton-and-kerry-discuss-us-role-world-stage