The Mythbusters TV show tried something like this with interesting results.
They built a section of sewer tunnel, including standard concrete pipes, vertical shafts and steel manhole covers, and attempted to blow off the manhole covers by filling the sewer with methane. There was som additional jiggery pokery by filing the tunnel with scrap, for reasons which will soon be clear.
The initial test shot was something of a disappointment, because the explosion was rather small. The end result was the steel manhole covers did get blown off, but only when a stoichiometric air-fuel ratio was achieved, including fans to mix the air/fuel into a uniform blend, and the junk in the tunnel to assist in creating turbulence to ensure mixing. The detonation wave did travel down the tunnel and up the audits, blowing off the manhole covers to a considerable hight.
This Mythbusters episode demonstrated the absurdities of the concept. First, the entire tunnel system needs to be filed with a stoichiometric air-fuel mixture of methane and air. The mix needs to be uniform throughout and there need to be no wear points for the detonation wave to escape, like the manhole audits in the Mythbusters episode. Considering the effort they needed to do this stunt in a small, artificially ideal tunnel, the idea that a uniform mixture will be present in an active sewer system spanning thousands of miles would require massive preparation. Then, the entire system needs to be sealed to prevent the explosion from escaping. Finally, you need to consider that a massive pipe 300 feet underground is already going to be extremely strong in order to resist the static pressure of all the earth over top of it (not to mention the tamping effect of the overburden).
Effectively, you might blow off manhole covers in select areas where ideal conditions have been created by accident, and possibly damage toilets and other water infrastructure connected to the sewers if the shockwave travels up the water pipes, but aside form some localized damage to the interior finish of the tunnels, the net effect will be "not much".
To develop a large amount of damage, active measures are needed. Terrorists dumping entire tanker trucks of propane into the sewers and igniting it using a timer to achieve the desired stoichiometric air-fuel ratio could collapse some sections of the tunnel. More determined terrorists will simply climb down and fix explosive charges on the tunnel directly. And firing a nuclear device in the tunnel should have interesting effects as the shockwave propagates through the various branches of the tunnel.