The tunnel that runs under railroad tracks in Saginaw appears to be an historical one. It is single lane but you will NOT be be walking IN the tunnel. The geocache is located at the east end. There is a side road where you can park to get out of the way.
You are looking for a large plastic jar that has some swag in it. You should access it from ground level and there is no reason to get close to any train tracks. You may have to stretch a bit to reach the cache. It might even be in sight from the ground. Cautions - Don't go near the tracks above, watch out for traffic in the tunnel especially if you any little geocachers along for the find.