I remember reading a spec sheet on this a while back, IIRC there are 20 FM's and 10 AM's piped into most (not all) of the I-93 Big Dig (O'Neill) tunnels via leaky-cable...probably Andrew NF-2DS or a derivative (that's a guess, but an educated one). So there are several signals "left out" of the rebroadcast scheme. There was some minor controversy because WFNX wasn't chosen; the selection was done by Arbitron ratings and WFNX didn't make the cut. I think that's why none of the non-comms were rebroadcast, but later WBUR might've been added. I don't remember if WBUR is still rebroadcast or not but I think it isn't, which is a shame.
Actually the OFC signals are tough pulls in a lot of the tunnels precisely because they're directly beneath OFC. Those towers on top are only so tall, so the skyscraper itself "shadows" a lot of the ground around it. And those are four-bay arrays (well, two for WFNX) so there's a fair amount of "focusing" of the signal, reducing top and bottom signal lobes quite a lot.
Based on when I've driven through the tunnels, you can get WERS and, to a lessor degree, WHRB in the northern half of the tunnels but not the southern. I don't know about WFNX. The Pru stations all come in fine but they're all rebroadcast, IIRC. I rarely could get WZBC down there, though...nor WMBR...must be a difference in car radios.