Ahhhhh ... Grimston Tunnel has special memories for me. As a 16 year old (1982/3) I walked all the way through this in winter. It was pitch black all the way through as it was a dark, wet, gloomy winters day and it was not possible to see the "light at the end of the tunnel" for most of the way through. I remember it as having a slight curve as every so often we walked in to the side and got covered in soot (though maybe the curve was just us walking on the uneven ballast and not going in a straight line. No torches or phones (1) so no light of any sort.
We thought it was disused when suddenly my friend said: "There's a train coming". I said "yeah right" and carried on. He insisted there was a train coming. I looked up and sure enough two yellow lights heading towards us. We darted in to a little alcove at the side of the tunnel which had a huge bucket in it. I booted the bucket out as hard as I could so we could shelter there. A train passed by slowly with a single carriage behind it full of engineers with head phones on. We legged it. The train stopped. We legged it some more all the time running in to the side and getting covered in soot. When we got to the entrance we tried climbing up the embankment but kept slipping down as it was all so wet and boggy. Eventually we got home covered in soot and mud so got a right rollocking after all the "what the hell has happened to you?". Happy days!
One other thing I recall was in the pitch black every now and again we would walk on drain covers that clanged loudly and felt unsteady compared with the ballast. Quite unnerving.
Thanks for the pictures :-)