The Fluid Loader/Unloader Augments are used to check what locomotive/stock is passing overhead.  Right click on a track segment to apply the augment.  It can be filtered by right clicking on the augment with a locomotive or rolling stock item.

The fluid loader will accept fluid from any pipe and store it in a small internal tank that it dumps into any stock which is overhead

The fluid unloader outputs fluid to any adjacent pipe or tank.

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