"one in fed by a small spring (probably only a few gallons a min flow rate) and has a natural pinch point so would only require a small dam (maybe 75ft at most) but it is also near a railroad (figure it may be high hazard classification)"

Step 1 is ALWAYS to determine what the regulations require/allow.

In my state (Kansas) the hazard classifications are based in part on the height of the dam and the acre-feet of water that is impounded.

I think your first location sounds better for a pond. My advice would be to find out the regulations, and then to design your pond for your conditions AND to fit just under the hazard thresholds.

I would also build the pond at a lower elevation than the spring (if viable). When a spring is in the side or bottom of your pond, then the spring both giveth and taketh away. You probably want the spring to trickle into the pond above your full pool water line.

Good luck on your new project!