Complex sentence: The tunnels themselves are still standing and despite no maintenance in decades are still structurally sound-ironic since the PTC is considering bypassing the deteriorating (yet still active) Allegheny Mountain Tunnel in the near future.
Split into simpler sentences: The tunnels themselves are still standing and despite no maintenance in decades are still structurally sound. This is somewhat ironic since the PTC is considering bypassing the deteriorating, yet still active, Allegheny Mountain Tunnel in the near future.