All the old buildings around here including the house I grew up in were made from hand hewn beams of hemlock. The old house was made around 1820 as the barn was. Keep them dry and they'll stay straight and strong. In the barn the rafters were made from young hemlock split down the middle using a pit saw I assume and the flat side went up for flat surface for the purlins and the round outside of the log is still seen today when looking up in the barn. The old barn is mostly straight and so is the house. Not bad for a bunch of northern hillbillies almost 200 years ago!
If I remember right, hemlock wasn't as bad at pitch buildup on the blade as other species like red pine and larch/tamarack.
Many folks like it for barn siding and roofing purlins/slats and such. If I had a bunch I wouldn't hesitate to use it for anything I needed, except for ground contact. This would be black locust.
As to your question I wouldn't use it for bridge decking unless it was temporary. Strength wise it'll be fine but it won't last long in the elements. I would, however, expect it to last 3 years. This is temporary in my eyes. This is just opinion - I've never used hemlock for bridge, blocking, road matting, etc. purposes.