Thanks for your question about your Robinia that loses leaves in the summer. From the photos it looks like the leaves that are turning yellow are the older ones. Is this correct? And have you checked it thoroughly for any signs of insects or diseases that could be stessing it? I will assume so in this answer.

Losing the older leaves is a normal reaction of Robinias (and other trees) to stress; in the summer it is a natural response to drought stress. Robinias are very drought tolerant, but that doesn't mean that they stay 100% green all summer - losing old leaves is one of the ways they cope with drought. They're known as rather messy trees, partly for this habit, and also their tendency to drop small dead branches. (They are also prone to root suckers, not something you would want in a garden railroad, although if this cultivar is grown from tissue culture it should be less likely to do that). So, it is entirely possible that this is the normal habit of this tree in summer, and that you can't change it

However, you say you have tried both increasing and decreasing the water with no effect. This makes me wonder if something else could be going on that is preventing it from getting the all water it needs. When you planted it, did you make sure the roots were spread out, or were they knotted up tightly in the pot? If it was very root bound, the roots may not be growing out into the surround soil. Also, if you planted it with the potting mix around the roots, that can dry out and be hard to re-wet, and could be keeping moisture from them. (Of course, the roots should be far beyond the initial root ball by now). The fact that this began two years after planting makes me think it is likely, since that's about the time it would be really growing strongly and be feeling the effects of a bad root system.

Another possibility could be something in the soil around it. Are there underground obstacles, or work that was done in that time frame that might have damaged roots? Is the soil very compacted? These could also be hampering root growth.

Hopefully this will give you some possibilities to pursue. If none of these suggestions are true, and you love the tree, I suggest a leaf blower to keep things tidy!