In Seattle, underground tunnels were shared by trains and buses until March 2019[1], here's an video showing it in action, a bit older, (possibly before there were any trains?): https://m.youtube.com/watch?v=etT_wVuKy_I
Why not use buses in those tunnels as well? Maybe there is no traffic jams above ground?
[1] https://en.m.wikipedia.org/wiki/Downtown_Seattle_Transit_Tun...