An Adventure I Did Not Have

Today, I did not have an adventure, although there was one practically offering itself to me on a silver plate.

On my way home, I loosely follow the rail tracks leading through the small town where I work to the bigger town where I live.
At various points, those rail tracks are crossed by bridges.
One of the narrower bridges was blocked off when I walked past today, and there was a rather big hole in the road in front of it.
Something in the hole caught my eye, and I went back to have a better look.

What I saw were three or four concrete steps, leading from an underground room which had obviously been there all along and was now exposed, into a tunnel just about big enough for an adult to crawl through it, and that tunnel seemed to go right underneath the now blocked-off bridge.
The underground room was empty apart from common building debris, and the steps were clean; the tunnel was a black square with nothing visible in it.

What was the purpose of both the room and the tunnel? Who built it there, and when?
The bridge itself is not very old; it was, if I am not mistaken, erected in the 1950s or 60s, so the room and the tunnel are most likely from the same period.

The road block was only about hip-high, and I could have easily climbed over it and down into the hole to investigate the mysterious tunnel.
As a child, I am quite sure I would have done it.
Now, though, I heard the group of cleaners that had just come out of the big office building I had walked past a few minutes ago, almost right behind me.
And it was late, I had been at work for almost 10 hours and was starving, my stomach constantly reminding me with loud rumbles that I should really go home and get something to eat.

My tummy won.