After upgrading minecraft to 1.5 and also railcraft to the compatible version I noticed that my tunnel bore sometimes stops (permanently) without any reason. I tried to reproduce the problem. It seems to happen when there's no block to mine but also no rail it can drive on. In this case the tunnel bore just has to lay down a track and continue.

However the problem doesn't exist in all cases so I created a new world which has this problem and uploaded to http://www.file-upload.net/download-7805644/TunnelboreProblem.zip.html so you can reproduce the problem.

Minecraft 1.5.2
Railcraft is up-to-date (Railcraft_1.5.2-7.3.0.0.jar)
Forge: minecraftforge-universal-1.4.7-6.6.0.497.zip
used McPatcher 2.4.5_02 to install forge and its own mods

You are sure there is no lava nearby or anything? Or massive drops ahead?

Yes, there's nothing like this. Inside of the uploaded example world I placed the tunnel bore on the surface. It just has to put tracks onto the grass and drive on it.

Interesting fact: The tunnel bore continues when I'm jumping into the bore head.