Q. Dear Rabbi. If one travels by subway or bus during the morning of Tisha Beav, does one have to remain standing or can one seat before Chatzot? (midday).

A. Kovetz Halochos (p. 221) rules that on both cases one may sit down, since it is more secure and one is not necessarily doing it for pleasure. (See similar p’sak by Horav C. Kanievsky zt’l in the Chazon Ish name).
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a