I have the same problem… thanks for describing how to demolish them. It would be helpful if the devs could make demolishing tracks and tunnels as easy as it is to convert them to high-speed/electric track. It seems like all they’d have to do is to apply the code from that tool to the bulldoze tool to make it much easier.
It would be nice if it was easier to place an extra track along an existing tunnel. Sometimes it works, and sometimes it tells you there is a terrain conflict.