Great analysis and I'll chime in with some commentary from what I know about the area in question, the properties, and some history.
I'll include a video from a YT user which shows the right-of-way from Pittsburg, IN, west of Delphi to the end of the Monon High Btridge to the east. I used it as a guide for the bridge and the area around it, right after the murders. Over 33,000 views to date, I take it some other people found it over the last 3 years, too ;-)
No, the bridge is not needed strategically. It would be a centerpiece of the trail system. The bridge was necessary for a train route, beginning perhaps a century or more ago. Maybe locals would know where those tracks extended. I assume it curled the mile west through Delphi proper. Toward the south I have no idea.
The line ran to Frankfort, IN, north of Indy. Another bridge built by the same company in the 1890's over Wildcat Creek is at:
40°28'02.0"N 86°37'52.1"W
It's the same RR line. West of Delphi the line would have curved to the NW towards Chicagoland.
Google shows the two bridges are roughly a 16 minute drive from each other using U.S. 421. Wildcat Creek is another tributary of the Wabash River watershed. The bridge was washed out during a storm in recent years, the center section of the bridge is gone now.
The routing must have dictated crossing the ravine in that area. It was strictly a locomotive bridge so no use for pedestrians or to connect sections of the city, per your example of Europe. The first time I visited Europe decades ago I was amazed at how vital and interesting the bridges were. The shopping bridge in Florence comes to mind first. Also the great variance of bridges in Copenhagen. I could keep going...
I'm not even sure if that small enclave of Delphi behind the bridge even existed when the train tracks were built. Those homes are more widely spaced than Delphi proper. Freedom Bridge over State Road 25 now serves to connect one side of Delphi to the other side. Delphi proper is on the northwest of 25 while the bridge is on the southeast side. From what I've read the bridge trail renovation plans include an extension of the trail all the way to center Delphi itself. Right now it is possible to walk that route but it would be clumsy along the east/west main road through town. I doubt many people do it.
In the video below it shows Milroy Crossing on the east side of the Freedom Bridge, it's roughly where the official trailhead is for the Monon High Brodge Trail by IN 25, a short walk from where the CPS bldg was located. The significance of this is a Milroy family owned the land the railroad ran the line through, so they must have made an agreement for a bridge to be built so Mr. Milroy and his workers could move animals, equipment, crops, etc., underneath the RR line. Samuel Milroy Rd. is named after him.
The only other families which would have had land along there on the north side of the creek where the RR went through would have been the M family and the Girard's. A USGS photo file I have shows a house down by the creek in the 50's, the current trail to the creek was the driveway to that house back then.
C.R. 625 becomes a private drive near to the bridge, as many of us know. Along C.R. 625 are a mix of post-war homes, mostly, built between 50's to the 70's, mostly. Some newer homes were built since the railroad line was abandoned, anyone with Google Earth on their computers can go back through archived photos to see this. Prior to that, the land was farms and some was owned by CSX Transportation.
There are other functional bridges in Delphi. I saw one at Canal Park that did serve to unite small communities on two different sides. I believe that was called Wabash canal. But at Monon High the creek does sort of an S pattern underneath the bridge, if you can envision that. The girls were killed near the top left swirl of the S. Then the middle of the S would be directly underneath the bridge. Then the creek loops around to the right (north) once beyond the bridge.
Crossing the creek in that area is hardly a daily concern. That is a very low populated section of a very low populated town. State Road 25 crosses the creek not far from Freedom Bridge. That road would account for virtually every crossing.
Translated to Falling Down-ese, my $.02: Nobody would have been along that section of creek along the creek bed, even on a mild February day.
Prior to 2014, the original right-of-way for the RR still existed where Freedom Bridge crosses IN 25, now. C.R. 300 had an easier path through that area, between Andersons and Delphi, yet it still would have been the same, sleepy rural road it is today.
Scroll lower at this link and the layout of the city is visible. Monon High Bridge Trail would be far right center of the map. You can see how isolated it is from the remainder of the town. I like that map because it shows the layout of the old railroad lines. As I suspected, it looks like the railway route through Monon High looped west to connect to other main lines in Delphi proper, and then exited with one track northwest near Canal Park and toward Pittsburg while another track curled southward near the current site of Trailhead Park. BTW, that's Pittsburg the Indiana version and not Pittsburgh, PA roughly 400 miles east:
Delphi Canal Center
Video showing a walk from Pittsburg to the MHB:
Another $.02 from me: My hunch is BG knows all this stuff, he knows that whole area in question very well.
JMO