That is the [i]Association of American Railroads[/i] Track Loading Vehicle. Manufactured from the frame of a retired SP SD45X, it contains a novel single axle truck in the middle that can be used to test a variety of conditions including track strength (the wheelset can exert lateral force on the rails and measure their deflection) or wheel profile performance (a wheel with a distorted flange or tread can be intentionally misaligned in order to observe derailment tendencies. The long wheelbase minimizes the affect of the supporting trucks on track strengh. The adjacent passenger coach houses test personnel and instrumentation. In this photo the single axle is raised in the travel positon.
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