Camp Atterbury, along with Camp Shelby in MS, trains National Guard before deploying overseas. With over 32,000 acres, they likely have more available to the units on site than they would have at the High Bridge area.
Good point on the training acres being available on far larger bases and the possibility of SWAT type police training. But....
it is not the number of acres that could potentially interest the military regarding the nature center. Rather, it would be the rare type of rail way bridge. The fact that it is no longer in rail use could make it even more attractive as training would be less disruptive.
Here's a wild card speculation.
Who would know how to detect sounds in the woods and pull off a complicated maneuver?
Army soldiers are taught these skills.
There is a training facility approx. 2 hours from Delphi - Camp Atterbury.
I wonder if they have ever used the area surrounding Monon High Bridge for exercises?
I don't think this possibility has been brought up before. And at this point, no possibility should be discounted.
As side note, most soldiers are not taught "woodsy" skills beyond a brief orientation in basic training. Rather, such skills are limited to Special Forces, light infantry, and to a lesser extent, mechanized infantry.
As to units using the nature center and bridge for training, such occasions are very rare (at least in the past). Rather, most units would use a simulated location on a military base.
An exception would be Special Forces units that truly need to practice on a specific type of terrain feature (say, a high railway bridge) with only the real thing being acceptable. As a result, Special Forces do use non military areas for training on occasion.
The training could be with the entire unit, or a "walk through" by officers and senior NCOs. I imagine, however, that if SF needs to train on a certain feature, they would train the entire unit. This would probably result in the nature center being closed for a day or two.
In short..... I would think the odds of the bridge being used for a SF training scenario would be low, but still very possible as there dont appear to be many such bridges around. Accordingly, the possibility should be "ran down".